HE WROTE The Da Vinci Code, which became one of the bestselling books in history. For an encore, Dan Brown is serving up another literary mystery, though not the one his fans had hoped for – what on earth has happened to his next book?

Almost five years have passed since Brown’s epic account of Vatican skulduggery and long-hidden secrets stormed to the top of bestseller lists around the globe, earning its author an estimated £125m.

Word of a sequel began to emerge in 2004, when Brown’s US publisher hinted that a new book would appear the following year. Brown has since revealed that he is working on a story about the influence of freemasonry in Washington DC in the mid-19th century.

Robert Langdon, the scholar-adventurer who was played by Tom Hanks in the 2006 Da Vinci Code film, is due to return in the new novel that was provisionally entitled The Solomon Key.

“For the first time, Langdon will find himself embroiled in a mystery on US soil,” Brown wrote on his website last year.

Since then there has been near silence from Brown, his agent and publishers about when the book might appear. “It is currently unscheduled,” confirmed Alison Barrow of Transworld, Brown’s UK publishers.

Brown’s American publisher, Doubleday, sparked a fresh wave of speculation last week by hinting that Brown had “a very specific release date” in mind.

“When the book is published, readers will see why,” Stephen Rubin of Doubleday told The Wall Street Journal.

The wait is proving both long and frustrating for a book that many publishers on both sides of the Atlantic hope will help fill the blockbuster void left by JK Rowling and the end of the Harry Potter series.

Brown was the UK’s most successful author in 2006, when revenues from the Code and three earlier books earned £11.5m, according to Nielsen BookScan. Yet the inevitable slowdown in sales of a book with at least 80m copies in print slashed Brown’s UK income last year to £1.3m.

The absence of a successor to what publishers have dubbed the “Da Vinci Lode” has inevitably sparked speculation that Brown is suffering from writer’s block or has run into trouble with his freemasonry research.

“There are famous examples of authors having trouble with a follow-up to success,” noted John Sutherland, emeritus professor of modern English literature at University College London and a Sunday Times critic.

Sutherland cited Margaret Mitchell, who never published again after writing Gone with the Wind, and JD Salinger, who followed Catcher in the Rye with a few short stories and nothing else after 1965. Thomas Harris, author of the Hannibal Lecter crime novels, needed 11 years to produce a sequel to Silence of the Lambs.

Sutherland speculated that authors in Brown’s position might be suffering the “equivalent of stage fright”.

“It’s the feeling Dickens had that you go up like the rocket and come down like the stick, and it may well be that Dan Brown feels this is his ‘down like the stick’ moment.”

There have been sporadic sightings of Brown undertaking his freemasonry research and speculation on the web that he is planning to do for masons what The Da Vinci Code did to Opus Dei, the secretive Vatican sect.

Akram Elias, grand master of Washington’s masonic grand lodge, said last year that Brown “had a contact with us but then cut it short. We are all sitting around waiting for his book to come out, but no one knows what he is going to say”.

Barrow last week insisted that it was “not unusual” for an author to be slow in delivering a book. She noted that Brown had plenty of distractions, including a high-profile plagiarism case that he won in Britain in 2006. He was also involved in the film of the Code and a forthcoming film version of Angels and Demons, an earlier book.

“As far as we are concerned he’s at work on a sequel and it will be published and we will be publishing it,” Barrow said. “There is never any clause from publisher to a novelist that they have to deliver at a certain time. We would not impose such a thing on a contract.”

There is speculation that the key date in Brown’s new book may be July 4. In addition to being America’s national independence day holiday, it was on July 4, 1848, that a group of freemasons – whose numbers included President George Washington – laid the corner-stone of the Washington Monument, the 555ft tall obelisk that towers over the Mall across from the White House.

Brown said in a court filing for his plagiarism defence that he regarded writing as “much like playing a musical instrument; it requires constant practice and honing of skills”.

Publishing insiders surmised last week that he may be obsessed with avoiding the factual errors that were uncovered in The Da Vinci Code, or that he may have become bogged down in a doomed effort to avert widespread criticism that he is not a very good writer.

“Or perhaps the Vatican may have sent out a hit squad,” noted Sutherland.

Additional reporting: Sara Hashash

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