wo years and 25 million copies later, Dan Brown, the author of "The Da Vinci Code," has all but gone into hiding.

Gone are the days when he could sit undisturbed in the Grand Gallery of the Louvre, sketching out the murder scene that opens his blockbuster novel. He has stopped taking commercial flights because of the commotion that usually accompanies him, with people lining up in the aisle to get his autograph on books, cocktail napkins, even the occasional air-sickness bag.

He has given almost no interviews over the last year, immersing himself instead in researching and writing the follow-up to "The Da Vinci Code," which will again feature Robert Langdon, the familiar Harvard religious scholar, and will be set in Washington and focus on the secretive world of the Freemasons.

"I have no idea how real celebrities handle their fame," Mr. Brown, 40, said last week in a rare written response to questions submitted to him by e-mail message. "I'm just a guy who wrote a book, and it still can turn into a circus at times when I go out in public."

His retreat from the public eye comes as expectations for his next novel grow bigger every day, as do sales of "The Da Vinci Code," a thriller that long ago morphed from a best seller into a cultural phenomenon.

Since its release on March 18, 2003, "The Da Vinci Code," Mr. Brown's fourth novel, has sold roughly 25 million copies in 44 languages around the world, including nearly 10 million hardcover copies in the North America. That is 10 times the average sales of industry titans like John Grisham and Nora Roberts, making the book one of the fastest-selling adult novels of all time. While most books move into paperback within a year of their original publication in hardcover, Mr. Brown's publisher, Doubleday, still has not scheduled a paperback release of "The Da Vinci Code."

Starved fans, meanwhile, have snapped up everything else Mr. Brown has written: his three earlier novels, which produced barely a ripple when they were published, have now sold more than seven million copies, according to Nielsen BookScan. Based on traditional rates of author royalties, Mr. Brown has probably earned close to $50 million in the last two years from sales of his four books in the United States alone.

"In some ways, my life has changed dramatically," Mr. Brown said, as when he arrived at the airport in Boston to catch the shuttle to La Guardia Airport - only to realize that he had left his driver's license at his home in New Hampshire. "Fortunately, the guy behind me in line had a copy of 'Da Vinci Code,' " he said. "I borrowed it, showed security the author photo and made my flight."

In other ways, Mr. Brown said, life has not changed. "My writing process, for example, remains unchanged," he said. "I still get up at 4 a.m. every morning and face a blank computer screen. My current characters really don't care how many books I've sold, and they still require my same effort and cajoling to persuade them to do what I want."

Not all of the reviews of Mr. Brown's works have been adoring, of course. The two most popular books, "Angels & Demons" and "The Da Vinci Code," have attracted intense criticism from religious commentators and even the Roman Catholic Church. Last week, an Italian archbishop was dispatched on the Vatican's behalf to debunk "The Da Vinci Code" and its theory that Jesus and Mary Magdalene married and had heirs. Cardinal Tarcisio Bertone, who was in charge of doctrinal orthodoxy for the Vatican before becoming the archbishop of Genoa, called the book "a sack full of lies" and urged Christians not to buy or read it.

The publicity has only amplified the pressure on Mr. Brown to produce a spectacular follow-up. While best-selling authors are always important for a publishing house, they have become the lifeblood of an industry in which sales of general-interest books have been essentially flat for two years.

When Mr. Brown's next novel will be published, however, remains an open question. Doubleday executives say they do not expect to see a manuscript until sometime next year.

There are hints that the pressure to repeat his success might be wearing on Mr. Brown. Long an author who worked in private, Mr. Brown now talks with his editor, Jason Kaufman, often once a day, sometimes twice - far more often, Mr. Kaufman said, than when the pair worked together on Mr. Brown's three most recent novels, including "Deception Point" and "Angels & Demons."

"We go over every plot point and twist," Mr. Kaufman said. "I function as a sounding board for him."

Stephen Rubin, the publisher of Doubleday, part of Random House Inc., said he was not at all concerned about when Mr. Brown will finish his next book.

"Why would I ever put pressure on Dan Brown?" he asked.

But while Doubleday had signed Mr. Brown to a two-book, $400,000 contract before publishing "The Da Vinci Code," the second book under that contract will now not come so cheaply. Heide Lange, Mr. Brown's agent, said in an interview that she had renegotiated the contract to include at least two more books and compensation that is commensurate with the success of "The Da Vinci Code." Neither she nor Doubleday would comment on the details, but the price is undoubtedly in the millions of dollars.

Many publishing industry executives think the risk of a disappointment is minimal - that the pent-up demand ensures that Mr. Brown's next book will be huge.

"He has an enormous fan base now that is willing to snap up anything he writes," said Tom Dwyer, the director of trade books, or general-interest titles, for Borders Group, the national bookstore chain. "Just look at the sales of his previous titles."

In addition to the unusually large promotional effort by Doubleday, the subject matter of "The Da Vinci Code" - Catholic Church intrigue and mysteries hidden within Leonardo's most famous works of art - helped advance its sales, making the book both compelling and easily accessible to readers, said Antoinette Ercolano, a vice president in charge of buying general-interest books for the Barnes & Noble chain.

The book made its debut at No. 1 on the New York Times best-seller list on April 6, 2003, and has remained on the list for 103 weeks, 51 of them ranked No. 1 and never dipping below No. 5. With that success has come some of the trappings of fame, which make it a challenge for Mr. Brown to maintain his privacy even when he tries. Not long after "The Da Vinci Code" was published, he got a call from Steven Tyler, the lead singer of Aerosmith, who read "The Da Vinci Code" and liked it. Mr. Brown and his wife, Blythe, have long been Aerosmith fans, and Mr. Tyler offered tickets to a concert.

The performance, it turns out, was filmed as part of the John Travolta movie "Be Cool." In the film's concert scene, there in the front row, shouting adoringly and clapping along, is Dan Brown, getting more screen time than most of the band and even some of the film's actors.