The CIA, the Freemasons, the Mormons and an unfinished pyramid on the US dollar bill are all expected to figure prominently in Dan Brown’s next adventure when it is published this year or early next.

Fevered speculation has been building among the author’s legions of fans since it emerged that hints to The Solomon Key were incorporated into the design of The Da Vinci Code’s American dustjacket.

With worldwide sales of more than 40 million copies, including 4 million in Britain, The Da Vinci Code and its blend of conspiracy theories, shadowy secret societies and thrilling adventure has already spawned an industry of its own.

A film version of the novel, starring Tom Hanks as the Harvard symbologist Robert Langdon, and Audrey Tautou, who played the title role in Amélie in 2001, as the French cryptographer Sophie Neveu, opens in May.

Bookshops are overflowing with parodies and with works promising to examine the issues raised in The Da Vinci Code. A further spate of books is attempting to pre-empt the subject matter of the sequel. In Secrets of the Widow’s Son, for example, the journalist David Shugarts purports to offer an “explorer’s field guide to understanding the main themes, ideas, symbols and historical issues which author Dan Brown will most likely utilise in The Solomon Key”.

A veteran of The Da Vinci Code industry, he and his team of researchers believe that Mr Brown has chosen Washington as the location for his next novel after setting Robert Langdon’s previous adventures in Paris, London and Rome.

In The Guide to Dan Brown’s The Solomon Key, Greg Taylor suggests that the new book will refer to the Masonic and Utopian views prevalent among the founding fathers of the United States and to the Skull and Bones society, the secret Yale brotherhood to which both candidates in the 2004 US presidential election once belonged. He also highlights conspiracy theories linking the unfinished pyramid on the US dollar bill to the Masons.

Doubleday, Brown’s publisher, has posted a “webquest” on the internet as part of its advance marketing strategy for the new book. The webquest challenges budding codebreakers to unravel a series of puzzles starting with the ciphers and symbols that are “already in your possession”.

The site says: “Disguised on the jacket of The Da Vinci Code, numerous encrypted messages hint at the subject matter of Dan Brown’s next Robert Langdon novel.”

A faint grid reference written in reverse on the cover leads, with an adjustment of one degree, to a sculpture called Kryptos in the courtyard of the CIA’s headquarters in Langley, Virginia. Kryptos is covered in about 1,800 letters of code, much of which is still a mystery despite its location at the workplace of some of the world’s shrewdest cryptographers.

A further clue on the jacket is visible with a magnifying glass. Some of the lettering describing the plot is in bolder type than the rest. When read separately from the other words the letters read: “Is there no help for the widow’s son?” Those words, a Masonic call for help, have been linked to Joseph Smith, the founder of the Church of Jesus Christ of Latter-Day Saints, known as the Mormons.

He started to say them as he fell to his death from a window after he was shot and fatally wounded by the mob who stormed his prison cell in Carthage, Illinois, in 1844.

Brown is reluctant to betray too many details but he has said that he grew up surrounded by the “Masonic lodges of our fathers” and confirmed that his next novel would be set “within the oldest fraternity in history, the enigmatic brotherhood of the Masons”.

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