Eragon, a fantasy novel written by Christopher Paolini, sits at number three on the New York Times bestseller list for children's books, out-selling all but the most recent of the Harry Potter adventures.

That bare fact is impressive enough, but behind it lies one of the most extraordinary publishing stories of the year, of a book which began as a 15-year-old's attempt to entertain himself and was published and marketed by the author and his family before being discovered by one of America's best comic novelists. It was later sold to a major publishing house for a reported advance of $400,000.

Knopf Publishers has so far printed 250,000 copies of Eragon, the first of a trilogy telling the story of a boy who finds a magic blue stone that hatches into a dragon. Together, the pair set off to take part in the war against an evil king who killed the boy's family. The book has been compared with the works of Tolkien and Philip Pullman.

'Eragon was the book I always wanted to read,' said Paolini, who has never attended school but was taught at home in Montana by his mother. 'I was bored and looking for a project. So I started a trilogy. The first draft came so easily, it was like a dam had broken.'

After two more drafts, he showed the manuscript to his parents, Talita and Kenneth, a former publishing company executive. 'Talita read it and said, "Ken, you really need to take a look at this." I was amazed,' Paolini told the New York Times.

The family edited the 500-page manuscript before publishing it themselves last year. The initial print run was 10,000 copies, most of which sold thanks to the salesmanship of the Paolinis. 'We have a very acute understanding of how to market a book,' said Paolini Sr, who contacted hundreds of schools and bookstores to ask if they would host a reading by his son.

Christopher made more than 150 appearances across the United States, dressed in leather boots, black pantaloons, a red swordsman's shirt and black beret. 'The first time I spoke in public was at my local high school, which I never attended,' he recalled. 'I find out they're going to cycle the whole school through in three batches. It's a farming community and there are all these ranchers' sons, and there I am standing on stage in medieval costume. I was petrified.'

Among those who read the self-published edition of Eragon was the teenage stepson of novelist Carl Hiaasen, who was fly-fishing in Montana with his family last summer. Hiaasen alerted his editor at Knopf, which then made the Paolinis an offer to buy the rights to the book.

The final version of Eragon was published this summer with only minor changes to the original text and a new cover to replace Christopher's self-designed dust jacket. The author, who is now 19, has started work on the second volume of what he has decided to call his Inheritance trilogy.

He has declined to say how the story might end, although, asked what he intends to do once all three books are completed, he said: 'I might go to college. Or I might take a vacation and have a nervous breakdown.'