Hogwarts School of Witchcraft and Wizardry, to give its full title, combines the endless history classes, illicit dormitory feasts and freezing games lessons of the classic fictional boarding school, with the rather more unusual world of three-headed monsters, flying broomsticks and the blackest of black magic; and the slip from the mundane to the bizarre is so cleverly done that readers find themselves believing wholeheartedly in this impossible, intoxicating world.

The creator of Hogwarts meets me in a cafe in the centre of Edinburgh. There are lots of children about and a herd of decorative elephants, but no obvious wizards. Yet Joanne Rowling must sometimes wonder if a magic wand has been waved over her life. Her first book, Harry Potter and the Philosopher’s Stone, published last year, has garnered the gold award from the Smarties Book Prize, a six-figure advance from an American publisher, and, last month, the children’s book prize in the British Book Awards. Seldom has a first-time author, and particularly a children’s author, been so successful so quickly.

Rowling’s head may still be reeling from the excesses of the award-night celebrations, but otherwise she appears to keep her feet firmly on the ground. “I did find it difficult in the summer, after the American rights were sold,” she admits. “I was writing the second book, and I froze for about two weeks. I felt as though every word had to be worth a certain number of dollars.” The panic passed, however, and Harry Potter and the Chamber of Secrets - as magical a read as the first book - is due out at the beginning of July.

Rowling has, she says, always written. Two unfinished novels for adults are hidden away, and she shrieks at the thought of their ever seeing the light of day. She started writing about Harry Potter in 1990, sketching out the basic history of a downtrodden boy who suddenly discovers he has magical powers. But then, she says, she spent a lot of time trying to find the right way to write for children.

“In the end, I realised you have to use your own voice. You can’t write down to children. You avoid using too many polysyllables and you miss out the lyrical prose; you can’t indulge yourself and set the scene for five pages, you have to get on and tell the story; but otherwise you write as you would for adults.”

The resulting books are direct, witty and seriously enjoyable for all ages. “It’s not like anything I’ve read before,” said Rowling’s American publisher, when he read the first Harry book. And then, in the next breath:

“It reminds me of books I read as a kid.”

There is indeed a timeless quality to the books, and the Kids’ Lit censors will find nothing to worry them.

Yet in the midst of this carefully-detailed dream world, Rowling filters in the subversive element which children love, and does not shy away from harsh reality. Harry, for all his wizard powers, must go back to his despised relatives at the end of term and return to his utterly powerless state. Rowling does not wallow in the cruelty of ordinary life, but nor does she shy away from it.

“Bloomsbury were brave to take it on,” she says. “Another publisher kept me hanging around for nearly a year while they made up their minds. I think they thought it was too long and too complex. It is 90,000 words which 50 years ago was the standard length for a children’s book. But now the marketing men have taken over and it has to be 40,000 and not a word more. But there must still be kids out there who if they love a book want it to keep going.”

Keep going it will. The story is just beginning as far as Harry Potter is concerned. Rowling has seven books planned, one for each year of Harry’s schooling, and even has the final chapter of the last book written - “I wanted to know where I was going.”

She is famous for having written the first book in a cafe while her baby daughter slept in her pushchair, and when I walk into this cafe she shuffles away sheets of paper covered in a mesh of boxes and names - clearly the plotting of the third book is an almost mathematical problem.

She looks ahead with horror to the day she will finish the seventh book and say goodbye to Harry. “It’s going to be like a bereavement,” she says. “Not only because Harry will leave an enormous hole in my life, but because there is an awful lot bound up in the books, particularly the first one. It was a tumultuous period of my life.”

She is referring to her arrival in Scotland, having separated from her Portuguese husband, with a baby and no obvious means of supporting herself. It is a testament to her determination that instead of sinking into despair, she wrote herself out of that difficult time.

She readily admits that the book’s escapism was a response to those dark days. Even now, she says, it is wonderful at the end of a tiring day to be able to dive into that world of trolls and dragons, secret potions and invisibility cloaks. Her daughter, now four, begs her mother to read her the book, but so far Rowling has refused. “I told her it was too frightening, and she said ‘But Mummy, I’m not frightened of anything.’”