(FORTUNE Magazine) – The Da Vinci Code, by Dan Brown, has been on sale for nearly a year now, and--assuming no unexpected plot twists--it should become the fastest-selling adult fiction title ever by March 18, when its publication anniversary rolls around. Apologies to all the religious-thriller, art-history-driven reading groups out there, but, uh, how the hell did an unknown author writing on an obscure subject make publishing history?

First of all, if you've not read the book--red cover, partial picture of the "Mona Lisa"...come on, people, get with the dinner-party agenda--its not your traditional bestseller. No military technology (Clancy), no legal minutiae (Grisham), no forensic science (Cornwell). This one is pure Catholic Church conspiracy, word puzzles, and a villainous, self-mutilating albino named Silas. The basic premise rests on Brown's (hotly contested) assertion that the Church has suppressed the real truth about Jesus and Mary Magdalene for thousands of years.

The Da Vinci Code started on the road to unexpected smash success when a new Doubleday editor, Jason Kaufman, acquired the novel, hoping it would sell between 20,000 and 50,000 copies. (There are 6.1 million copies now in print; barring an unexpected wave of unsold copies returned by stores, that should soon vault the book ahead of the 4.3 million copies of The Bridges of Madison County sold in 1993.) When the manuscript was distributed in-house, however, it was a sensation. "Everyone went bat shit," says Doubleday president Steve Rubin.

The company decided to print a whopping 5,000 advance reader copies (ARCs) to distribute to reviewers and book buyers--a number on par with Little Brown's campaign for 2002's breakout book, The Lovely Bones, and 2001's The Corrections, from Farrar Straus & Giroux. One of those advance copies went to Sessalee Hensley, a fiction buyer at Barnes & Noble, who stayed up all night reading it. "The Doubleday salespeople came to us and said, 'This is a great book,' " she says. "And we came back to them and said, 'This is the greatest book.' "

Indeed, Barnes & Noble helped turn Da Vinci into a flat-out phenomenon: The company ordered 15,000, then 30,000, and finally 80,000 books--unheard of for an unknown author. The first Da Vinci printing, which Doubleday had originally planned for 60,000 copies, was reset to 230,000.

With such a big bet on the table, B&N made some unusual moves. It put the company intranet to work so that store managers could post reactions and share ideas about how to sell the book. Some stores had greeters assigned to tell people about Brown's novel.

Publishing insiders suggest that Barnes & Noble may be throwing its weight behind new authors in order to regain ground lost to discounters like Costco, which dominate the market for name-brand authors. Says B&N CEO Steve Riggio: "We tend to take risks on titles we think have potential--that are not considered guaranteed blockbusters--while the mass merchants wait to see what we do, then react."

Whatever B&N's motives, its early commitment made a huge difference. "What they did was...insane," says Doubleday's Rubin. "I was terrified--230,000 copies for an author nobody had ever heard of?" For its part, Doubleday sent out another round of 5,000 ARCs, bringing the total to 10,000--far and away the largest advance mailing ever in publishing.

The rest, as they say, is history. On the day before publication, Janet Maslin of the New York Times published an effusive review declaring The Da Vinci Code to be "blockbuster perfection." The book debuted at No. 1 on the New York Times bestseller list, and it has stayed on the list for 47 weeks. ABC ran a special on the subject in November, and Columbia Pictures has announced plans to make the book into a movie, directed by Ron Howard. As for Doubleday, it's planning a new mass-market paperback house with The Da Vinci Code as the first title. The only problem: The book still sells so well in hardcover that it might not be ready for paperback anytime soon.

And Brown? He wasn't available for comment. He's working on a new book. --Grainger David