When the French architect Marc Levy recounted elaborate bedtime stories to his young son every night, he couldn't have dreamed that they would one day turn him into a multi-millionaire. The novel Levy based on those stories, If Only It Were True, has sold 230,000 copies and will soon be translated into 28 languages. On its release in France in January, the book went straight to the top of the bestseller list and remained there for eight consecutive weeks. It is still hovering near the top of the charts.

But the real puzzle behind the book is how an untranslated French manuscript written by a virtually unknown first-time author triggered nearly every major Hollywood studio into a bidding frenzy for screen rights. The result is that Levy's book has been bought up by Steven Spielberg's Dreamworks studios for $2m.

"The funny thing is that I've never really had any literary ambitions," says Levy, the 38-year-old director of a large architectural firm in Paris. "It always amused me to scribble down ideas and invent all kinds of characters for my son. Sometimes I pretended to take notes during professional meetings, when in fact," he smiles, "I was somewhere else."

Levy's success story began two years ago when his sister Lorraine, a television scriptwriter, discovered a half-finished manuscript and persuaded her brother to develop it for an adult audience. The novel was immediately accepted by the respected Paris publisher Robert Laffont Editions, and programmed into its bestseller collection for the coming season.

Set in San Francisco, where Levy lived as a student, If Only It Were True tells the story of Arthur, an architect who moves into an apartment and discovers in his closet the spirit of a young woman, Lauren, who has been in hospital in a coma for six months. They fall in love, but Arthur is the only one who can see her and his friends and colleagues believe he's gone mad. When doctors threaten to unplug the life-support machines, he kid naps his beloved's body in a stolen ambulance and takes her to a remote hideaway. Lauren miraculously recovers, but has no memory of her passionate affair with the stranger at her bedside, and Arthur has to court her all over again.

"Basically, it's about a guy who falls in love with the letter and not the envelope," says Levy. "It's something like Saint-Exupéry's idea in The Little Prince that you can only see clearly with the heart."

Susanna Lea, who handles international rights at Laffont, says that when she first read the manuscript, the story immediately struck her as movie material. She prepared a brief English summary of it for last October's Frankfurt book fair. "We could never have predicted it," she says, "but hours later, the phones were ringing off the hook - all the studios were calling in the middle of the night to make an offer."

No one was more surprised than Levy himself, at home in Paris, stuck in bed with Epstein-Barr virus. In the auction for the film rights, the bidding narrowed down to two studios: Fox 2000 and Dreamworks. "It wasn't a just question of money because the others were offering the same amount. I've always a had tremendous respect for Spielberg's work," he says.

Shortly afterwards, Levy was flown to New York for a five-hour meeting with Spielberg to discuss the fine points of the novel, which the director hadn't read. Once the movie option had been finalised, Susanna Lea yielded to the storm of foreign publishers clamouring to buy the book rights. "We paid seven figures for it," says Judith Curr, president and publisher of Pocketbooks, a division of Simon & Schuster.

So doesn't anyone object to the fact that literature can now be bought and sold at a colossal price solely on the basis of a minimal plot summary? "The fact that the scouts got excited after reading two paragraphs doesn't diminish Levy's novel," says Curr. "People get the essence straight away. I call it the 'tingle effect'."

So far, Levy seems undaunted by his sudden launch into literary stardom and considerable fortune. But he has decided to put architecture on hold and settle in London to work on his next book. And although Spielberg has appointed him "creative producer" of the film, Levy prefers not to say who he would cast. "It's like taking my child to see Santa Claus," he smiles. "I'm not about to ask how they're going to wrap the gifts."

• If Only it Were True is published by Fourth Estate at £10. To order a copy for £7 plus 99p UK p&p freephone 0800 3166 102 or send your order with a cheque payable to The Guardian Culture Shop, to 250 Western Avenue, London W3 6EE.