Susanna Clarke's first novel is like no other: an original, elegant fantasy. And it was all hard work, she tells Eileen Battersby.

With 400,000 copies of her first novel in print in the US, where she has just completed a 14-city tour, an exhausted Susanna Clarke returned to England to discover that her big book, adored by Americans, is set to become very big. Jonathan Strange & Mr Norrell, a fantasy picaresque like no other that has come before, is original, elegant and sophisticated and will appeal to all ages. It is also very funny. The humour is important to Clarke, who says she isn't sure if she's naturally funny. "I'd like to be, but am I funny? I don't know."

If the Englishwoman appears suitably eccentric, slightly odd, even formidable in her photograph on the book's jacket, looking very much like a person who wears spectacles and is missing them, Clarke in person is instantly likeable, with a student-like aura. She is pleasant, soft faced, spectacle-wearing and rather pretty. Her expression tends to shift between amused, bewildered and thoughtful.

She is a northerner whose journey south, including her time at Oxford University and in London publishing, has left her accent somewhat flat and emphatic. Nowadays when she goes north she is told she sounds like a southerner. Yet her love of northern England is crucial to the novel. "My father was, and is, a Methodist minister, so we travelled around quite a bit through the north and the midlands," she says, and, yes, her accent is a mess.

Born in 1959, in Nottingham, she will be 45 next month, within a couple of weeks of Halloween, an appropriate birth date for the writer of an atmospheric novel about magicians. "Yes, well, it's a dark time of the year." She thinks about what she says, but there is nothing calculated about her replies. She just likes being accurate and is still alert to the unreality of the interview circuit.

As for the book, she says: "I love stories; I like big novels that have stories. Story is undervalued, don't you think? I read anything for story." And she expands: from comics to Sherlock Holmes. "I prefer 19th-century fiction to 20th or 21st century, and I felt it should be possible to write a 19th-century novel in the 20th century, which it still was when I started to write it."

Featuring on this year's Booker longlist was a pleasant surprise. "I never expected that, and it pleased me. It showed the book had been accepted." Missing out on the shortlist, however, was a relief. "I felt I would be asked if it was a serious book. What is 'serious'?" Justifying her book is not something Clarke should have to do. But British fiction trends, still transfixed by Margaret Thatcher's Britain, are not as welcoming to 19th-century fiction as might be expected. Historical novels and fantasy narratives seem to fall beyond the threshold of serious fiction.

Perhaps her book is best described as seriously good. Non-fantasy readers will enjoy the book as much as fantasy fans. Whereas the Harry Potter books, none of which she has yet read - "but I will" - have a contemporary setting and offer an alternative world, her book is a period work.

The just under 800 pages of Jonathan Strange & Mr Norrell took a decade to write. "I thought about it, dreamt about it, for six months and then began to write it in 1993, and I finished it about this time last year. At first I was going to write in the form of letters, but I abandoned that." When she began, most immediately inspired by Charles Palliser's The Quincunx (1990), an elegant and multilayered Victorian mystery tour that confirmed her belief that it was possible to write a 19th-century novel now, she had an idea rather than a plan.

Asked if she found it difficult to end such a long narrative - was the final sentence hard to write? - she sits bolt upright in her smart hotel chair and says: "Every sentence was hard to write; it was all very hard work."

Although she says it is a fantasy novel, and she is well aware that writers of fantasy and science fiction tend to be marginalised - "so I have no difficulty in saying it is a fantasy" - her book is a literary novel, elegantly written and drawing more on early-19th-century English social and political history than on special effects or gadgets. Among the many wonderful qualities that make the book stand out is the fact that, in writing a book about a magician's determination to revive English magic, she has created the sense of a Regency London in which magic is accepted as part of daily life.

Much of the appeal of the book lies in its normality: there is nothing showbizzy about the magic - no wands, no cauldrons, no monsters; only fairies, spells and the distinction between "books about magic and books of magic". The narrative evokes a lost tradition, a tradition that was a vital part of an entire culture. Clarke has concentrated on the contrast and similarities between men and fairies; the tension between reason and imagination. There is also a Faustian element: in order to bring a woman back from the dead Mr Norrell must secure help from a powerful fairy, who strikes a tough bargain.

Another strength is the characterisation. Unsurprisingly, Clarke has a soft spot for Mr Norrell, a grumpy and reclusive misfit magician who is frightened of people, mice and cats but is nevertheless brought out of seclusion by John Segundus's plan to revive English magic, setting out on a crusade to bring magic to London and help in the war against Napoleon. Norrell proves an interesting character, very real and very human in his faults.

As she writes near the close of the book: "Mr Norrell had many talents, but penetration into the hearts of men and women was not one of them." Jonathan Strange is likeable if totally caught up in himself, again a very plausible characterisation.

Some of the characters shine with Dickensian flair, such as the noble Childermass, and there are comic turns, such as Drawlight, a parasite and con man, but as the narrative becomes darker so do some of the players, such as Lascelles, initially an indolent man about town, who emerges as a killer.

Character as much as incident and plot drives the narrative, and Clarke achieves a sense of real people living through a dramatic situation created by several of them. One of the central characters is Stephen Black, a black servant who has the favour of his master yet becomes trapped in a spell. His outsider status becomes significant within the layers of the narrative.

Clarke also uses historical individuals such as Wellington and Byron. Historical detail adds texture to the fiction. Her sense of history is easy to explain. "My father was, is, a historian, and I've always loved it, so it felt right." But she did not study history at university. "No, I did PPE" - philosophy, politics and economics - "because at one time I had thought about becoming a journalist, and apparently that's the degree for getting into journalism."

Mention of her time at Oxford does not fill her with nostalgia. "I hated it. I didn't fit in. There were lots of reasons, not just the degree." She is the eldest of three children; moving about so much as a child left her constantly in the role of the new girl at school. "I tended to use books as my way of getting by. I love the way you can retreat into a book." She has read Tolkien - "he really knew what he was about. I knew his material while I made up much of mine" - Mervyn Peake, C. S. Lewis and Lewis Carroll. Without imitating any of them she has deferred to them yet remained true to her individual period world view.

These writers have clearly informed her style of imaginative storytelling. But the major influence as far as narrative drive is concerned must be Dickens. As for shaping the voice of the novel, which was always going to be written in the third person, she listened to that of one of her heroines, Jane Austen. "I like that wry tone: Austen can be very sharp, almost cruel. People see her as very polite, but there is such an edge to Austen. The style came quickly." She thinks about this, agreeing that Austen's measured savagery influenced the dialogue in Jonathan Strange & Mr Norrell, then adds: "I have drawn very definitely from Austen and Dickens."

If the actual narrative voice came quickly once Clarke began writing, how about the book itself? "I had had the idea I wanted to be a writer. And I remember saying I wanted to write a big book and was told I wouldn't have the perseverance." This is said without a trace of defiance. Since she began writing she has published seven short stories and novellas, including The Duke Of Wellington Misplaces His Horse and Mr Simonelli, Or The Fairy Widower. Both have appeared in US anthologies; the latter was shortlisted for the World Fantasy Award in 2001.

But her big book is the story for us. Clarke, who enjoys talking about it, admits as a lover of digression to having had a lot of fun with its elaborate footnotes, which confer a scholarly tone and are, as she says, John Uskglass included, largely invented.

"When I was in America there was a lot of interest in the footnotes. People wanted to know if they were true. I had to think about this about this and admit most of them aren't but some of them are true."

How important is England to her? Clarke peers at me, and there is a feeling of having arrived at the heart of her big book. "The north of England is very important to me. It is my England; I have a loyalty to it. That's why I began the story in York: you don't get more northern than that. I love the darkness of the countryside, the romance of it. When I was writing the book I often read Ted Hughes's Crow and all that about the stones and the rain. I have a romantic view of England."

Clarke speaks a natural, unforced and functional English; the language in which her novel is written has a convincing period tone: it is very good pastiche. "That's the way I wrote, that's the language." Her visual sense is well developed, and she remarks that metaphors and images sit easier in a fantasy work than in a conventional narrative.

Home is a small house in Cambridge. She and her partner, the writer Colin Greenland - "as in the country" - live in the town. "You thought I lived in a great country house?" Well, no, more like a romantic cottage on the edge of the moors. "I'd like that," she sighs. But for the moment they are in Cambridge, where she is working on a book that is not a sequel, "but it takes up the story a few years after this book ends".

Jonathan Strange & Mr Norrell is published by Bloomsbury, £17.99