Codes in common: Dan Brown's new novel draws many diverse people together – which, oddly, is the moral of his book

Boyd Tonkin finds few sensations, but signs of a more sceptical, self-conscious writer

"We invented a non-existent Plan, and they not only believed it was real but convinced themselves that they had been part of it for ages, or rather they identified the fragments of their muddled mythology as moments of the Plan... But if you invent a Plan and others carry it out, it's as if the Plan exists. At that point it does exist."

No one knows much about what the unfathomable Dan Brown thinks. Sometimes, I suspect, the world's best-selling writer of adult fiction must have sleepless nights in which he feels a little like the aghast book editors at the close of Umberto Eco's novel Foucault's Pendulum. Their elaborate joke or game about a hunt for arcane knowledge and occult forces has been overtaken by assorted plotters, theorists, crackpots and conspiratists whose faith imbues a diverting fantasy with the power to change, even to end, lives.

If you have a taste for the uncanny, nothing could be stranger than the accuracy with which Eco's follow-up to The Name of the Rose, published in 1988, offers an advance warning of the Angels and Demons and Da Vinci Code mania. Already aware of the neo-medieval currents swirling around popular culture in the 1980s, the Italian semiologist, critic and historian – Brown's Robert Langdon with humour, scepticism and sanity intact – had spotted the allure of "so-called Tradition, or occult philosophy": that "eternal and rather eclectic ramshackle structure, swarming with Knights Templars, Rosicrucians, alchemists, Masonic initiates, neo-Kabbalists". It sounds like a future syllabus for Brown himself.

Eco's analysis feels even timelier this week, as readers flock to snap up the million-strong UK print run (out of 6.5 million copies) of Brown long-delayed fifth novel, The Lost Symbol. Kate Mosse, whose own chart-toppers Labyrinth and Sepulchre took a Brown-tinged readership for esoteric historical mysteries into challenging new territory, comments that "the razzmatazz is, in part, to do with our - perceived - need for an 'event'. The first night, the premiere of a film or an opera, the big match, everything about modern media and arts is geared to making a splash. But reading is a private pastime, one best done alone and in quiet, so how else - other than turning certain publications into public events - to have that same sense of occasion?" For Mosse, "the fact that, in lieu of Harry Potter, there's another such event focusing so much attention on the world of books, is great."

Yet The Lost Symbol hints that those bated-breath fans should simply enjoy the ride rather than hanker after some final illumination or apocalypse (a term it takes pains to define correctly). Indeed, the novel feels as if the author has learnt from his weirdly prophetic critic. "Initiation", writes Eco, "is learning never to stop. The universe is peeled like an onion, and an onion is all peel."

Behind the uncovered secret, another secret must lurk. Behind the last veil, its lifting indefinitely postponed by the striptease of a chase plot, there hides not treasure but trivia. Or, as one of several highly self-conscious moments in The Lost Symbol (Bantam, £18.99) has it, "Each time you pull back a curtain, you face another". If Brown's latest breathless scoot through hermetic history has a moral, it might be to relish the shared journey without worrying too much about the arrival.

The book often sounds expressly written as if to upbraid secret-hunters and conspiracy-lovers. This frenzied hunt through Washington DC for the ancient wisdom of the Masons buried among its mystic monuments unravels itself as it goes along. Hardly a chapter passes in which Brown does not seem to deconstruct his own fame, and even his own fan-base. Typically, Langdon and Katherine Solomon, his scientific sidekick in a race around the Capitol and other landmarks to locate a "precious Masonic secret", examine the Great Seal of the United States on a dollar bill. Of course, they know that the note's symbolic pyramid "was popular among conspiracy theorists as 'proof' that the Masons held secret influence over our early nation".

Brown's novel accepts that influence among the Founding Fathers, sure enough, but downplays the secrecy. It presents, from Benjamin Franklin and George Washington onwards, a wholly friendly account of Masonry: "one of the most unfairly maligned and misunderstood organisations in the world".

The Masons will have no need to recruit anything like the Jesuit rebuttal squad sent out by the Vatican in the wake of The Da Vinci Code. One episode of hi-tech suspense in The Lost Symbol even turns on a maliciously edited recording of initiation rituals among the Washington elite that would give ammunition to "anti-Masonic conspiracy theorists". Heaven forfend.

The action, as it unfolds over a single hectic night in DC, has intricacy rather than complexity. Langdon imagines he has been summoned to give a lecture in the Capitol building, whose all-pervasive Masonic iconography gets a thorough going-over. In fact he has been lured to the city by "Mal'akh", a shaven-headed, tattooed – and self-castrated – villain who seems to have barged in from the latest James Bond set. He needs Langdon to decipher the meaning of a small pyramid adorned with occult signs, whose layer-upon-layer decrypt keeps our hero occupied. The CIA, needless to say, also wants to know these secrets. In the pocket-sized "off-the-chart IQ" shape of its security queen Inoue Sato, the agency has its own grim ways of solving the clues.

Meanwhile, our brooding baddie has kidnapped Langdon's mentor, and Katherine's big brother, Peter Solomon. This super-rich scholar (shades of Leigh Teabing in the Code, but much nicer) and upfront Mason has reached not the third but 33rd degree of initiation. Katherine, by the way, runs her own hush-hush lab in the bowels of the Smithsonian museum. There, in a purer and sillier SF vein, she experiments with "noetic" mind-over-matter science that aims to convert the collective power of human thought into a mighty physical force, "like a flock of birds, or a school of fish moving as one".

Plenty of furious Brownian motion ensues, with a shoal of red herrings, a pack of shaggy dogs and a maze of false trails. Significantly, this intellectual sprint to read the "map" of occult Washington switches between a quest for real locations and messages and a study of the "purely symbolic" meanings of Masonic lore. Strip out the fun and fancy from this entertaining, overblown romp and you glimpse a critique of the dogged literalism that bedevils so many Dan Brown acolytes.

"I've had a feeling," Langdon sighs at one point, "we're treating as reality a collection of myths and allegories." He views the notion of ancient wisdom about forgotten human powers locked in some "Masonic Pyramid" as "an 'archetypal hybrid' – a blend of other classic legends, borrowing so many elements from popular mythology that it could only be a fictional construct... not historical fact". Talk about a book that knows how to read itself.

As for the awesomely dumb Mal'akh, he comes across as the Brown fan from hell. He will imprison, torture and kill to find out that ever-elusive nugget of hermetic lore beyond the final veil. The paranoid reader incarnate, forever sniffing out "circles within circles", our eunuch avenger hides a (not terribly surprising) family shame of his own as he barrels his way towards "this country's greatest untold secret". Although we run a gamut of favourite flaky allusions – Isaac Newton the alchemist, the Rosicrucian Order, Albrecht Dürer's mystic numerology, even the coded references within James Sanborn's "Kryptos" sculpture at the CIA headquarters– every box of esoteric knowledge in The Lost Symbol opens to reveal not a treasure so much as a mirror.

The truth, in proper Masonic or even New Age fashion, turns out to lie within – and all around. This becomes a tale of mind over mystery as well as mind over matter. Which means that the denouement delivers both a stiff dose of non-sectarian uplift and, for sensation-seekers, less delight than they might hope. That might be Brown's point.

True, the splendidly nutty motif of Katherine's research into the power of shared mental states allows for some gold-plated mumbo-jumbo that links ancient mythology with modern cosmology. "The same science that eroded our faith in the miraculous," she gushes, "is now building a bridge back across the chasm it created". As for the Masonic theme, it dwindles into a multi-faith yearning for "a single universal consciousness". Our aproned brothers "on the square" figure as the unlikely harbingers of a rainbow-hued fraternity. Even Google, Twitter and Wikipedia take a bow as messengers of an enlightened age of togetherness, "multiple minds working in unison".

All this may look dull to the conspiratist brotherhood, but it sounds bonkers only in the most innocuous manner. Brown's final tricks – more sugary than sinister - also help to justify his own success. When do we ever see evidence of a "synchronicity of beliefs" that draws people of all creeds together with a single mind? Since Potter's passing, only Brown himself can perform that global alchemy with the printed word.

The Lost Symbol not only talks – at sometimes wearying length - about the wired world as "a big entangled web of information" that will let us live in mystic (or even Masonic) oneness. Hyped, sold and consumed across the planet, it offers itself as a foretaste of that very unity. The medium becomes the message. Reader, don't fret that this book never quite unlocks the ultimate secret of enlightenment. For better or worse, you hold it in your hands.