SUSANNA Clarke was sitting in the plush London offices of publishers Bloomsbury when it dawned on her just how big her book was going to be.

"It was when I first went down to see them and I realised how many people had actually read it," she says. "I kept bumping into people in the corridors and they would say how much they liked it. Even people in sales and marketing had already read the manuscript. I knew from my years in publishing that this was quite unusual."

Such was Bloomsbury's confidence in their new author that they published a print run of a quarter of a million for her debut novel, Jonathan Strange & Mr Norrell, compared with around 1,000 for the average first offering. Since then there has been a Booker prize nomination and a tour of America, not to mention the weight of being touted as the new JK Rowling. Now, to Susanna's obvious delight, the studio behind the Lord of the Rings trilogy, New Line, has snapped up the option for the film rights for a six figure sum and are said to be determined to make the movie a reality.

"It is inevitable if you write a book about magicians," she says, on being compared to the famed author of the Harry Potter series. "I don't mind at all. It's the sort of thing journalists and book sellers and publishers all say. But it's the readers who get the book in their hands who will be able to judge."

And it is rather a hefty book at that - the kind you could use to prop doors open. The novel follows the fortunes of Mr Norrell, a desiccated old scholar, who stuns the citizens of York by bringing alive carvings in its cathedral, and his impetuous young assistant Jonathan Strange. The two magicians find themselves drawn into the Napoleonic wars and discover they've conjured up more than they bargained for. The background to their story is a world when England was ruled by a magician, the enigmatic 'Raven King' John Uskglass.

It is a meticulous, intensely imagined piece of work, often told through detailed footnotes. And it took Susanna a staggering ten years to pen her 782-page tome.

"I'd been trying to write a novel since I was in my early 20s and they had all been abandoned," she says. "I got a fair way with one but I had a problem with just knowing how to structure a novel. It was only when I re-read Tolkien's Lord of the Rings when I had an illness that it kind of became obvious - I should be writing a book about magic."

The idea for a novel about magic set in a recognisable Regency England gathered fruit in County Durham at her parents' home. Susanna, 44, was born in Nottingham, and, as the eldest daughter of a Methodist minister, had a somewhat nomadic childhood, moving every few years. She studied at Oxford University and worked in various areas of non-fiction publishing in London before leaving to teach English to stressed-out motor company executives in Turin and Bilbao.

Returning to the family home, overlooking the North Sea at Blackhall Rocks, near Peterlee, she began to draw on her Northern surroundings and religious upbringing to create her magical world.

"I think if you are brought up with a religious background then you have a sense of the world outside the perceived world," she says. "It's very natural to you to think of heaven, hell and unseen things, and those being as real as the things which you perceive.

"The north has also made a big impression on me and I still consider myself to be a northerner. I feel that the roots of the novel are in the north. A lot of the preparatory work was done in County Durham, particularly on the Raven King and the medieval background.

Before Susanna began writing she enrolled on an Arvon Foundation creative writing course which was to prove valuable in more ways than one, as it was there that she met her future partner, novelist and reviewer Colin Greenland.

"My whole life is different because of that course," she smiles. "When we met, we just had lots and lots to say to each other.

But Susanna actually began writing her book in Cambridge, where she now lives. She would often dive out of bed at 5.30am and fit in a couple of hours before her publishing work. She would also scribble away at weekends. "It was a sort of obsessional thing," she laughs.

"If I didn't write at the weekends I would feel really rather uncomfortable. But if somebody had said to me it was going to take ten years to write, I don't think I would have had it in me to keep on going.

"It was very hard. There were lots of times I would sort of despair and say, 'that's it, I've wasted enough of my life, I'm going to give up'. But it felt like something I had to do, and the next day I'd be writing again. And I always had faith in the characters, I never got tired of them."

Her faith in them has now been rewarded. Strange and Norrell earned her a long-listing on this year's Man Booker Prize, a huge and well-deserved compliment for a fantasy writer.

"It was amazing," she says of her nomination. "If you're writing a fantasy novel in a literary style and it's got magic in, you really don't expect to get on the Booker long-list. Fantasy writing is a genre and like any genre it has to struggle to be taken seriously, so this has not only been good for me but for other fantasy writers."

Susanna says she has not aimed Strange & Norell at any audience in particular, but simply set out to write a book she wanted to read - a fantasy novel with a strong narrative and a bit of comedy thrown in. (She drew on Jane Austen's heroines and her humour to create some of her characters such as Arabella Strange). It is being touted as an adult book but at a recent signing in America there were all ages in the audience and it is difficult to believe that the young adult population won't succumb to its magical charm.

For now, Susanna is trying to fit in work on her second novel, once again set in the mysterious world of magic, around her hectic schedule.

"I'm going to stay in this world because I'm very interested in it," she says. "But it will be moved on a few years after Strange & Norrell finishes and it will be set in England. It won't be a sequel in the structured sense." Before then, fame and riches beckon, and possibly a film deal. Has she any idea which Hollywood actors should take on the roles of the reclusive Norell and dazzling Strange?

"There's a lot of interest but I don't want to get into that," she says coyly. "Apart from anything else I would find it really annoying if I was the actor. But it's the sort of thing my friends get together in pubs and talk about, they really enjoy casting people."

The last few weeks have seen a positive whirlwind of press and publicity events. As a lover of the northern countryside, Susanna says she is looking forward to a weekend of relaxation in Derbyshire.

"It will be a brief rest in all the madness," she smiles.

l Jonathan Strange & Mr Norrell (Bloomsbury, £17.99