We may assume that Dan Brown knew precisely what he was doing when he slipped an early reference into his fifth novel to “the Philosopher’s Stone” — in this case the original mythic alchemical device but also a clear nod to J. K. Rowling.

For only the Harry Potter books come close to matching the mania and marketing hype surrounding The Lost Symbol — 6.5 million hardback copies of which have been printed ready to strain thumbs on buses, trains and planes around the world.

This is the third adventure in capital cities for the Harvard symbologist Robert Langdon, who took in Paris and London in The Da Vinci Code and Rome in Angels and Demons. The cover artwork of The Lost Symbol, featuring the Capitol Building and a masonic key, has had hyperventilating Brownophiles speculating wildly since it was first seen in April. The novel has our hero summoned to Washington to give a lecture on architectural symbolism, thence sucked into a 12-hour national security crisis that takes in noetic science, the ancient mysteries and a giant pickled squid.

As with Langdon’s previous escapades, tourist-friendly secrets are hidden in plain view. We get the “deification” of George Washington, as seen in the Apotheosis of Washington fresco in the Capitol, and a mural showing him laying a cornerstone in “a full masonic ritual”. Expect Lost Symbol walking tours within the week.

Brown writes genre fiction but his Langdon thrillers are laced with art history as well as political and theological fact, making their dismissal as junk both patronising and misleading. If anything, The Lost Symbol is too heavy on the footnotes. The pace of the unfolding jeopardy — a kidnapped mentor, a dismembered, pointing hand, a ruthless tattooed ghoul — is slowed down, rather than dumbed down, by all the exposition. The balance between story, puzzle-solving and Open University course was well maintained in the previous two books; less so here.

Early on, Langdon mocks students for having visited Rome, Paris and London but not their own capital city. Brown seems to be saying to his US readers: “Langdon’s coming home.” He even manages a gag at the expense of his own novel’s long gestation, when Langdon’s publisher bemoans a lack of manuscript and asks him: “Why can’t you just stay at home and write?” Naysayers may simply ask, why can’t Dan Brown write?

It’s true, his style is as baldly prosaic as legend, but there remains a heft to his potboilers that is hard to imitate. He is better at conveying claustrophobia and breathlessness than, say, the explosion of a top-secret lab (“fragments of titanium mesh . . . droplets of melted silicon” etc) but the latter will make a juicier scene come the inevitable Tom Hanks movie, and the author knows this.

The Lost Symbol will not bring down the US Government with its sugared depiction of masonic influence (“When different cultures are killing each other over whose definition of God is better, one could say the masonic tradition of tolerance . . . is commendable”) but it will divert millions from more pressing matters. Since the hype was begun by fans, we have only ourselves to blame if the latest reconfiguration of a proven formula doesn’t live up to expectations.

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