Attempts by fans to tunnel into the tomb of Abbé Bérenger Saunière in southwest France, convinced that it held the Grail’s secrets, represent the sharp end of Dan Brown tourism, “Grail Trails” retrace the characters’ steps in Westminster Abbey and Temple Church, London, as well as locations in Scotland and France. At the Louvre in Paris, some visitors are even said to ignore Leonardo’s Mona Lisa to ask: “Is this where the curator was murdered?” Brown’s earlier novel Angels and Demons has spawned tours of Rome (where it was set) to see the Colosseum and St Peter’s Square. Irish theme pubs are also doing a roaring trade with parched American codebreakers. For the more independent of mind, Fodor’s Guide to the Da Vinci Code was issued in February.

RELIGIOUS REVOLT

Brown’s preface claim that descriptions of documents and secret rituals were accurate (they are not), lead many to believe Jesus’s relationship with Mary Magdalene was more than just conjecture. The Archbishop of Genoa organised a debate disputing the claims, calling them proof that anti-Catholicism was the “last acceptable prejudice”. Opus Dei — the Roman Catholic organisation portrayed as a secretive cult — issued a 127-page response after receiving daily e-mails asking why it was hiding the truth about the Holy Grail. “We would like to remind them it is a work of fiction and not a reliable source of information,” it said.

LEGAL WRANGLES

The spectacle of two of the three authors of The Holy Blood and the Holy Grail suing their own publisher, Random House, over claims that Brown had plagiarised their work may have seemed strange. But the publicity helped sales to rocket by 3,500 per cent on Amazon.com, pushing the 1982 book back in the top 40. Random House are now reissuing it in hardback and paperback. A conspiracy? The truth is in the bestseller chart.

GASTRONOMY

So, The Da Vinci Code has a special place in your heart — but what about your stomach? Stephen Lanzalotta, a baker, claims that you can loose weight by basing your diet on the principles of the Golden Ratio, or Phi: a mathematical formula used in the novel. His book, entitled The Diet Code, rather prosaically encourages a Mediterranean diet of bread, fish, cheese, vegetables, meat, nuts and wine (that’s 20 per cent protein, 52 per cent carbohydrates and 28 per cent fat, code-breakers). But do you really need a mathematical formula, or just the right aisle at Waitrose?

SPIN-OFFS

Dubbed “Brownsploitation”, the industry in books piggybacking on the success of The Da Vinci Code is now almost as big as the original itself. There are the inevitable similar novels (Raymond Khoury’s The Last Templar, Steve Berry’s The Templar Legacy, even the “Jewish Da Vinci Code” The Righteous Men, written under the pseudonym Sam Bourne by the journalist Jonathan Freedland); guides promising to “unlock” its secrets (Dan Burstein’s Secrets of the Code, The Rough Guide to the Da Vinci Code); and those debunking them (The Da Vinci Hoax and Truth and Fiction in The Da Vinci Code). But all of these are in better taste than the spin-off film The Da Vinci Load, a gay sex romp due out the same day as the Tom Hanks blockbuster.

POLITICAL UPSETS

Brown’s page-turner made waves in the world of politics when Ruth Kelly, the Education Secretary, was found to be a member of Opus Dei. The revelation did not go down well with critics, considering Kelly’s responsibility for sex education.

CODE-BREAKERS

The novel is steeped in code-breaking, so related games have become popular. Web-based quests (initially part of the promotional campaign) have spawned The Da Vinci Code Quest — an online puzzle involving 24 tests and promising “untold riches” for the winner. The inventor of another game became part of the books. John Langdon invented “ambigrams” (writing a word so that it can be read upside down, back to front, or in a mirror). Brown wove them into Angels and Demons, and named its hero after him, also giving him a role in The Da Vinci Code. His name is now known internationally and he is inundated with commissions for ambigrams.

CHART-TOPPER

The Da Vinci Code has now sold 40-million copies worldwide (4 million in Britain) and has been translated into 44 languages. It is the seventh most bought book of all time (although it is still some way behind the 6 billion sales of the Bible). All this despite being savaged by the critics, one of whom wrote: “It was the only book I had on a trans-Atlantic flight, and I seriously considered asking them to turn the plane around.”

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy