At one minute past midnight, the fifth Harry Potter book will hit the bookshops. In an exclusive interview, J. K. Rowling tells Ann Treneman how she has finally come to terms with celebrity, and how marriage and her children have made her happier than she has ever been.

JOANNE KATHLEEN ROWLING is a happy woman these days, and it shows. She greets me at the top of the staircase at her home, babe in arms. His name is David and he is round and soft and cooing. We all go into the front room and there, on the shelf, is the other baby in her life: a 1kg doorstop that is the fifth book in the Harry Potter series, Harry Potter and the Order of the Phoenix.

“It is big. Very big,” she says. “I didn’t dare do a word count.”

So how big is big? After all, Harry Potter and the Goblet of Fire, the fourth book, was 636 pages.

Joanne peeks at the last page. “It is 766 pages. When I finished it, I thought, Oh my God it’s bigger than Goblet. I knew already it was but I thought, well maybe it’s slightly bigger and then I spoke to my editor at Bloomsbury and she said, ‘You know how long it is, obviously?’ And I said, no, I don’t actually. And it was a quarter of a million words.” Her voice goes almost to a whisper. “I nearly died.”

Don’t you have an editor who cuts things, I ask rather abruptly.

She laughs and takes on an actor’s voice: “Don’t you have an editor? Does anyone ever try to stop you!” She reverts to her normal voice. “Yeah. Of course they do. But they truly felt that the information contained in the book was necessary.”

This is the third time I have interviewed J.K. Rowling. The first was in 1997, after the publication of the first book, Harry Potter and the Philosopher’s Stone. She was a rising star with no idea of the galaxy into which she and Harry would soon soar. “I never dreamt this would happen,” she said then, when sales reached 30,000. “My realistic side had allowed myself to think that I might get one good review. That was my idea of a peak. So everything else really has been like stepping into Wonderland for me.”

Wonderland indeed. Three years later, in May 2000, we met in an Edinburgh hotel room. She had just finished Harry Potter and the Goblet of Fire and was quick and funny and nervous, smoking five Marlboro Lights in two hours and talking like a machine-gun at full rattle. At that time she had sold 30 million copies, a film was coming out and merchandising deals were brewing. Her wealth was estimated at £15 million but her life, which revolved around her daughter and writing and friends, had none of the gloss that money can bring.

Fast forward three years. Rowling has now sold almost 200 million books and is worth an estimated £280 million. She is wealthier than the Queen and is listed as the 122nd richest person (and the ninth richest woman) in Britain. Some people would revel in these facts, flashing them about like a diamond in sunlight. I doubted Rowling would: the last time we met she .denied she was famous and said her only major purchase had been an aquamarine ring that she called her “No One Is Grinding Me Down” ring.

I was curious to meet her again and see how she had changed. It is true that I had not seen Joanne showing off her lovely dining table in the pages of Hello! magazine or anything like that, but you never know: money and fame can corrupt as much as power. Facts are few. She is 37 now and married Dr Neil Murray, an anaesthetist, 18 months ago. Jessica, her daughter from an earlier marriage, is almost 10, and David was born in March. The family has houses in Edinburgh, Perthshire and London.

Her main home is in Edinburgh and that is where we meet. For some reason I had decided that she might be a minimalist - a hangover, or so my logic went, from the days of poverty. Wrong. Her home is vibrant with colour and patterns, and the front room busy with books and photographs. It is not a showcase but a lived-in family home. Apparently there is a dog somewhere in the house. Certainly there is a baby in the room who provides a gurgling backing track for the interview.

J.K. Rowling looks terrific. She gave up smoking three years ago and, as she is breastfeeding, has even had to forego the Nicorette. She explains this as she reaches for a pack of Wrigley’s and advises me to buy shares in the company. The interview, as events tend to be when tiny babies are involved, is the result of meticulous planning. She spent all weekend wondering how she was going to get the baby fed and changed and herself presentable “with all my buttons done up properly” at the correct time.

It takes one minute to see that she has changed. Definitely. She is more relaxed, her edges rounded off. The machine gun has been replaced by a lower and softer voice, though her chuckle-laugh is the same. I say that she seems different, calmer.

“I’m loads calmer. Yes. Loads. I think I’m loads happier now, which would make me calmer.”

Well, I say, you weren’t the last time we met.

“But you saw me probably during the worst time. The last time you interviewed me was not a happy time. Writing Book Four was an absolute nightmare. I literally lost the plot halfway through. My own deadline was totally unrealistic. That was my fault because I didn’t tell anyone. I just ploughed on, as I tend to do in life, and then I realised I had really got myself into hot water. I had to write like fury to make the deadline and it half killed me and I really was, oh, burnt out at the end of it. Really burnt out. And the idea of going straight into another Harry Potter book filled me with dread and horror. And that was the first time I had ever felt like that. I had been writing Harry for 10 years come 2000 and that was the first time I ever thought, Oh God, I don’t want to keep going.”

Rowling, who had the idea for the seven-book Harry Potter series on a delayed train to Manchester in 1990, had not taken a break since she began writing in earnest as a broke single mum. She wrote mostly in cafés then. When she finished one book she began the next immediately, sometimes on the same day. And so, fresh from producing Harry Potter and the Goblet of Fire, the fourth book, she felt immense pressure to start the next.

It was not the first time she had felt the strain of the deadlines. “The first thing that I did when I finished Prisoner of Azkaban was to discuss repaying the advance for the next book.” I look shocked at this. “Yes, you can imagine. People were a little bit shaken, I think. I said: I want to give the money back and then I will be free to finish in my own time rather than have to produce it for next year.”

And now, after Book Four, she again told her editor that she couldn’t make such a tight timescale for the next book. “Because I knew I couldn’t do it. Well, I probably could have done it. Because I do work hard. I COULD have done it, but the book would have been lousy and I would have then collapsed completely and said: That’s it, no more. I can’t do it any more. So, I said this to them.” Her publishers told her to produce the book at her own pace.

She had a break from Harry but kept on writing because, as she says, “I have to write”. She wouldn’t say much about what she was writing, except that it was “totally for me” and a story. Like a novel? “Yes,” she says. It is unfinished.

The break lasted the best part of a year. “I was also really conscious - and I didn’t need anyone to tell me this - that I needed to stop and I needed to try to come to terms with what had happened to me. I had to really try to cope with what had happened because I wasn’t coping. I wasn’t coping at all. For a long time people would say to me, ‘What is it like to be famous?’ and I would say ‘I am not famous’. Now this was patently untrue. It was the only way that I could cope with it, by being in so much denial that I was virtually blind at times.

“I always felt like I was racing to catch up with the situation. So I could cope now with the fact that I was being doorstepped but I couldn’t cope with the fact that they were now going after my private life. I was always several steps behind. I couldn’t grasp what had happened. And I don’t think many people could have done. The thing got so huge.”

She is always asked why Harry Potter has been so successful. “And I cannot answer that question. I can’t. It sounds coy. It sounds disingenuous. I never think of it like that. I think it would be dangerous for me to think about it like that, to sit down and analyse it, to decide why. It would be an exercise in navel gazing. It would also possibly lead me to deduce that I was doing certain things right and maybe certain things I should drop and if you start writing like that...”

From your head and not your heart, I say.

“Exactly. Then I think you are lost. And I would certainly be lost if I stopped enjoying it. And ultimately I need to do this. I mean, what is the point? I could have stopped writing four years ago and we would have been fine financially. So I’m not writing for the money. I could really do without the fame. The only point is to satisfy myself now and out of loyalty to the fans.” And Harry too, I say.

“Absolutely. When I say for me, it is for Harry ... being true to what I know will be his end.”

How would you describe your feelings about fame?

“I never wanted it and I never expected it and certainly didn’t work for it and I see it as something that I have to get through, really. It does have nice aspects but for me, personally, probably the negative outweighs the positive. And we are talking here about being famous as opposed to having the money because the money has obviously relieved me of an enormous amount of worry and it has made my children secure in the sense that I do know they will have enough to eat and so on. And that is what the money means to me.”

Yes, I say, but you are way beyond that.

“Absolutely. It went way beyond that.”

Is it odd?

“Yes, it is very odd. And you feel guilty about it. A friend of mine said to me the other day, ‘But I would just go in a shop and I’d just say I will have one of those, one of those and one of those in every colour. Why don’t you do that?’ But the fact is that once you can do that, you don’t really want to do that. The amount of stuff you actually want to buy, when you can, shrinks a lot. Whereas when I was completely broke, I would have bought anything.”

So you wanted to acquire things, then?

“Yes. Because I couldn’t. I just couldn’t. I mean, a new tea towel, I could get quite excited about. You think I’m joking!”

What do you mean about feeling guilty?

“It just seems, well, this came to me through doing the thing that I love doing most. So I suppose I feel that I haven’t suffered enough pain for it.”

I say that is not how it works.

“I know. I know. We all know it doesn’t work like that. The world is completely screwed up. When David was born I had a company sending me free Babygros. I found it quite upsetting and I actually got quite tearful at one point. I remember Jessica, if someone had given me free Babygros then, that would have been a very big deal. That would have made my whole week. It is just very unfair, isn’t it?”

Rowling says she loves to write, has to write, happy or sad, but that it is far easier if she is happy. The new book has been written during the happiest period of her life. She had already started work on it before her marriage on Boxing Day, 2001. I say it must have been thrilling to meet someone new. “It was incredible. I always wanted to have more children and I had reached the point where I thought, OK, I’ve been so lucky. I’ve got the books. I’ve got Jessie. I cannot complain and then this has been just amazing.”

Is it true, I ask, that you meet someone when you aren’t looking for them?

“Yes. Definitely. I did not expect to meet anyone, actually. I thought the baggage was too much and it is a truism that when you do get famous, it’s not that I didn’t meet anyone, it’s that I didn’t meet anyone I wanted to have a relationship with, much less marry. Of course, you do meet people but it tends to be those who are very keen to approach you and maybe not those you would really want to meet.”

She says it is fortunate for both her and her husband that their careers are so divergent. “The night we met he told me he had read the first ten pages of Philosopher’s Stone on a late-night shift at the hospital and he thought it was quite good. And I thought that was fantastic. He hadn’t read the books. He didn’t really have a very clear idea of who I was. It meant that we could get to know each other in quite a normal way. I think he’s up to speed now, poor bloke. At the time he didn’t really have any idea about it all.”

She wrote most of the new book in Edinburgh and some in Perthshire. She no longer writes in cafés because people watch her and it makes her self-conscious. At home she writes all morning in her office, which is the size of single bedroom and the smallest room in the house, until she gets hungry, about 12.30pm usually. She breaks for a sandwich, then goes back to the computer until Jessie comes home from school (she has not had a nanny since becoming a two-parent family). They walk the dog, a Jack Russell. She makes tea. Neil comes home. Depending on how tired she is, she may write more in the evening.

One day a week is spent doing “charity stuff”. She has a charitable trust and is the patron of several groups, including one for single parents and the Multiple Sclerosis Society Scotland (her mother died of the disease in 1990 at the age of 45). I say that I believe she gives quite a lot of money away anonymously and she stares at the carpet, lips pressed.

Rowling became pregnant mid-book and knew she wanted to finish before the baby came. “I was getting bigger and bigger and bigger and then, just before Christmas, I realised I had finished the book and it was the most amazing thing. An incredible thing. It actually really took me by surprise. I was writing the last chapter, rewriting bits of it as you do, and then I wrote myself to the end of a paragraph and thought: Oh my God, I’ve finished the book! I couldn’t believe I’d done it.”

I make some comment about how long it is and she says: “It’s hysterical. They went in one day from saying, ‘She’s got writer’s block’ to saying, ‘She’s been self-indulgent’. And I thought, well, what a difference 24 hours makes.”

The “they” in that sentence is the press. She resents the idea that it has been reported that she had writer’s block almost as much as she resents the pressure of a deadline. She admits to being “too thin-skinned”. “But that is who I am and I couldn’t do the books if I weren’t who I am.” She was genuinely distressed by the accusations, levelled by the American writer Nancy Stouffer, that she was a plagiarist and she celebrated when a New York court ruled last year that she was innocent. She is fierce about protecting Jessica’s privacy, never using her in publicity or going with her to film premieres. She rarely talks about her, although, when I ask why she bought the London house, she laughs and says she had been staying at Claridge’s and “my daughter was getting a bit too used to room service”.

It is easy to forget, sitting in this warm and light-filled room, about the darker side of Potter mania. But it is out there. Some people are obsessed with the idea that her books are teaching children about evil and magic and believe Rowling is a witch of some kind. “I found death threats to myself on the net,” she says, describing how she was looking for something when she found herself on a Potter-hater site. “And then halfway through this message board I found, well, people being advised to shoot me, basically. Which was not a nice thing to find. It is bizarre.” She sighs. “But what can you do?”

“Fame is a very odd and very isolating experience,” she says. “And I know some people crave it. A lot of people crave it. I find that very hard to understand. Really. It is incredibly isolating and it puts a great strain on your relationships.” Most of her friends have been doorstepped and offered money by newspapers for their story and that makes Rowling feel guilty.

Her views on some journalists are embodied in Rita Skeeter, a character who, when last seen, had become a beetle and was trapped in a jar. “I have a fascination for Rita and I have grudging respect,” says Rowling. “She has the rhino hide that I would quite like to have but haven’t. And you’ve got to admire her tenacity and ingenuity. But I wouldn’t like to meet her.”

It is difficult to do an interview on a book that I have not been allowed to read. She sympathises, but neither does she give much away. “This book is a bit of a departure. Harry is very angry. Very angry. And he’s angry for most of the book. But I think that is fair enough given what has happened to him and that he hasn’t been given an awful lot of information. So it’s not a very gentle tale. And there is a nasty death in it as well. Nasty because it is someone I care about as a character.”

She adds: “This time it is someone I consider to be a main character.” She cried when she wrote the death scene, as she did twice when writing Harry Potter and the Goblet of Fire.

Harry now “is very much in puberty, having as easy a time of it as I did.” And that was? “What I was, I wasn’t sure and I don’t think anyone else was either! I just think it is a very confusing time. Yes, he’s very confused in a boy way. He doesn’t understand how girls’ minds work.” I say that, at age 15, boys don’t normally say anything at all. She laughs and says Hermione is more than happy to fill in all those silences with her advice.

“This time Harry really, for the first time, does have a relationship of sorts. The emphasis very much on the ‘of sorts’. That was really fun to write, actually. I think you will find it painful. You should find it painful, it is painful, but it was such fun to write. Poor Harry! What I put him through.”

She has already started to write Book Six. “I started it when I was pregnant. That was a different situation because I knew I didn’t have to so that immediately meant that I wanted to! You know, the absolute reverse of Goblet of Fire. And I’m also in a very lovely position. Contractually, I don’t even have to write any more books at all. So no one can possibly write that I have missed a deadline because I actually don’t have a contractual deadline for Six and Seven.”

So you have freedom, I say.

“I do have freedom. I want to spend some time with David because I didn’t have him to hand him over to a battalion of nannies. But I do really want to do Six and Seven.”

Surely, I say, Six will be shorter. And she agrees. “Seven, on the other hand, will probably be massive ...it has been such a massive part of my life now. I can see myself being really scared to let go of it. I will probably reach the end of Seven and think, I’ll just tweak it a bit more, I will just tweak it a bit more. The fact that I will have finished will be extraordinary.”

But isn’t the last chapter of Seven already written? Yes, she says, it’s hidden away. In a secret place? “Guarded by trolls.”

Doesn’t anyone know?

“I’ve told no one. Literally no one. If you ever hear anyone claim that they know what happens in the end, they are absolutely lying. I’ve never told anyone.”

Maybe if you got drunk ...

“I would never tell anyone. I just know I wouldn’t. You couldn’t get me drunk enough!”

It is time to go. David has exhausted his mobiles and swings and we have talked for one and a half hours. This interview is very different from the previous one, and it seems to me that in the past three years Joanne Rowling has grown up. She has faced her personal demons about fame, money and insecurity. She has balance in her life and now, in addition to everything else, freedom. It is a heady mix, certainly a Wonderland, but she will tread softly there. “I am the kind of person who expects Mr Catastrophe to be lurking around the corner because he often has been. “I try to strike a balance between being very grateful for what has happened - because I am so hugely grateful for it - and I am terrified of hubris because I think it could all go wrong tomorrow.”