KATE MOSSE is getting used to being gazumped. First there was her name. Then there's the plot of her third novel, Labyrinth, a ripping yarn set in France about a search for the Holy Grail. She had been researching the story for years before a certain Dan Brown had some success with a similar subject matter. Ever felt, I ask her, like shouting, "I was there first!"?

"Both of those things have only benefited me," Mosse says. "If I shared a name with a murderer or tyrant, that might be damaging, but Kate Moss is a beautiful young woman." And The Da Vinci Code? "I first saw it at an airport just after delivering the manuscript of Labyrinth to my publishers. I thought I was going to have a heart attack. I was worried people would think I copied Dan Brown. But having read his book, Labyrinth isn't similar in tone and intention. Th e success of The Da Vinci Code was such that people who enjoyed it were looking for something in the same vein. And I don't believe Labyrinth would have sold five million copies worldwide without The Da Vinci Code."

Being beat to the punch like this is not unusual. While researching her latest novel, Sepulchre, Mosse discovered six different operas about El Cid played in fin de siècle Paris, the period in which her story is set. At least this time Sepulchre's plot - a Parisian innocent discovers diabolical goings on while on a rural retreat - hasn't been swiped before publication.

"There is no such thing as coincidence," a tarot reader tells Sepulchre's heroine. Mosse, on the other hand, doesn't believe in fate, she tells me in her Sussex home ahead of her appearance at the Edinburgh International Book Festival. She has, however, had her run-ins with what looks like destiny. Take, for example, the way in which she met her husband, Greg, an old school friend she hadn't seen in 20 years until, Sliding Doors-like, she happened to run into him on a train.

An author and creative writing teacher himself, Greg put his wife on the road to literary success in a more oblique way. "The key inspiration for me is landscape," Mosse explains. "Twenty years ago, we bought a tiny home in Carcassonne, only because my mother-in-law has a friend who is a real estate agent there. He could have been an estate agent in Crewe or Cardiff. The second I stepped off the train, I thought I felt at home here. And I knew nothing about the region; I didn't even know France well. I started to research the place, not for a book, but because I loved it. We'd been living there on and off for ten years before a novel about a Grail story came to me."

As a young reader growing up in West Sussex, Mosse devoured books that evoked a strong sense of place. "I did spend a lot of time walking about the marshes where I grew up and finding them mysterious and exciting. I read a lot of unfashionable books - gothic novels, which were all about atmosphere."

However, her first creative endeavours were musical rather than literary. Volunteering for violin lessons solely to get out of maths, Mosse discovered she had an aptitude and was soon a member of a youth orchestra. On an exchange trip, she made her first visit to France - to Chartres, which would play a role in Labyrinth. Her best-seller is partly set in 1209 during the Albigensian Crusade, a crude power-grab on the part of France's northern barons, who sought to stamp out the south's independence. Their plan involved branding their opponents heretics, particularly the Cathars - a tolerant religious sect active in southern France.

Mosse herself has a family connection to the Crusades, the de Mosses having originally come over with William the Conqueror. De Mosses also participated in the Siege of Acre during the Third Crusade, stories of which her father would tell her at bedtime during her childhood.

"It affected the kind of stories that I liked, and it was also part of my relationship with my father," she says. "Those sorts of family things, they influence you as a writer in ways you don't realise."

After graduating from Oxford, Mosse went into writing, but not as an author; she joined the publishing industry. Ironically, it was something defined by Cyril Connolly as an enemy of promise - "the pram in the hall" - that led to her becoming an author.

"When I was pregnant with my first child, the book I wanted to read about pregnancy didn't exist. There were plenty of books about the health side of pregnancy, but not about the emotional side. And emotionally, I had a difficult time being pregnant; I didn't really like it. When I was pregnant for the second time and having lunch with a friend who was a literary agent, I said Here I am again, and the book still hasn't been written'."

Her agent called her bluff and asked why she didn't write it herself. At the time she had been looking to get out of publishing. She was writing reviews for the broadsheets and was one of the founders of the Orange Prize for Literature (she remains honorary director). She accepted her agent's challenge, and soon had a book deal with Virago. After Becoming A Mother was published, her musical background led Mosse to write The House, about the Royal Opera House in Covent Garden. A fiction editor noted she was good at teasing out people's stories and asked her to write a novel.

"So I wrote two novels (Eskimo Kissing and Crucifix Lane), very much having a go," she says. "But I didn't really find my voice as a novelist until Labyrinth. People often think Labyrinth is my first book. I have heard some people describe my first two novels as literary. They're not; they just didn't sell very well."

There's a joke in there, I say.

"With Labyrinth, I stopped worrying about what sort of writer I was supposed to be. I just went for it. Out of that I discovered what kind of writer I am, which turned out to be an adventure writer. A common mistake people who want to be writers make is that they assume the kind of books they like to read will be the kind of books they write. The truth is, you have a persona as a reader and your persona as a writer might be totally different."

In Labyrinth and now Sepulchre, Mosse has sold a phenomenal number of her well-bred yarns. Hokum they may be, but they have the sort of page-turning intensity that sets cash tills ringing around the world and belie the amount of work her fiction demands. Three years of research went into each novel. She took the unusual step of documenting the progress on her website. Some writers are secretive creatures; Mosse gave readers a weekly update on how her work was going. The fact both books are about secrets only adds to the irony.

Her adventures on the internet are all a part of the process for Mosse, who is keen to demystify writing, particularly for younger readers.

"A lot of people think oh, I couldn't be a writer'. By showing the process, particularly online, you can say to a much wider range of people you can be a writer'. It's possibly a naive hope that somewhere there will be an 18-year-old sitting at a job they don't enjoy, from a background where reading isn't a key part of the family, who will see the site and think I can tell a story like this'."

Part of the same rationale was at work when Mosse asked another blogger, Lily Allen, to judge last year's Orange Prize.

"The idea of inviting Lily came from the book trade's awareness that there is a lost group of readers," Mosse explains. "There's an enormous focus on children's reading, but children start to drift away from reading at 14, 15, 16 - not all, but some do - and don't come back to books ever. It's a black hole. What we were trying to do by asking Lily Allen to judge was to see if having someone younger talking about literature meant we got a young age group - a demographic the trade agrees is hard to reach - to engage with books."

Critics accused the Orange Prize of pulling a publicity stunt, and when Allen swiftly dropped out of the judging panel, scepticism turned to jeers. "It didn't work out, no doubt about it. We got clobbered by the press," Mosse says, sounding rueful. "But it doesn't mean we're going to stop trying to engage with younger readers." Amy Winehouse is still in with a chance then.

Mosse herself is in no need of the Orange Prize's pixie dust, not as far as sales are concerned. Interestingly, her readership is evenly divided between male and female, she says, if the audience at her readings are anything to go by. With Mosse's novels now translated into 37 languages, the day might be fast approaching where this author's name rivals that of a certain supermodel's.

Kate Mosse will be at the Edinburgh International Book Festival on August 19 at 4.30pm. www.edbookfest.co.uk