Sales of the book, which sees the return of fictional symbologist Robert Langdon, have already reached the highest level of customer pre-orders at retailer Waterstones since the release of Harry Potter author JK Rowling's adult fiction “The Casual Vacancy” last year.

“We think it's going to be the fastest and biggest-selling book of the year because Dan Brown is in a league of his own,” Waterstones spokesman Jon Howells told Reuters.

“The early word is that it's darker, which will whet the appetite for everyone because this is the fourth appearance of Robert Langdon and I think giving it that sort of twist, freshens it up for long-term readers,” Howells added.

Brown, who is famous for his love of puzzling codes, conspiracies and other symbols, said the subject matter for the book was so vibrant and horrifying that it helped do a lot of the work for him.

“I'm not writing about the masons and ancient histories, which is kind of ethereal. I'm writing about Dante's vision of hell,” he told Britain's Sunday Times newspaper.

“It wasn't until the 1300s and this version of Inferno that it became terrifying. Dante has had enormous influence on the Christian view of hell.”

The author, who said he tries not to read reviews of his books, also spoke of his unique way of dealing with writer's block, by suspending himself upside down to help get his creative juices flowing.

“It does help. You've just got to relax and let go. The more you do it the more you let go. And then soon it's just, wow,” he said.

Brown's religion-themed mystery novel “The Da Vinci Code,” was published in 2003 and was made into a hit film starring Tom Hanks. It spent more than a year atop the New York Times bestseller list.