There's a parody Twitter account called @BanksyIdeas. It consists of possible works to be undertaken by the pseudonymous street artist, related in a relentlessly ironic tone. "Stencil of Richard Dawkins arm-wrestling Jesus, yeah?" "Stencil of the Blue Peter ship, yeah? But it's being boarded by a group of Somali pirates." It's one of the few examples of Banksy-related ephemera not mentioned in Will Ellsworth-Jones's Banksy, which is a shame, as he'd surely have something to say about what it might indicate about either the fickleness of hipster taste or the plummeting of the artist's "subversive" credentials; his work is now often as dated in its minutiae as an episode of Trigger Happy TV.

Yet his stock hasn't fallen, as the success of his knowingly self-parodic film Exit Through the Gift Shop makes clear. This disarmingly written biography purports to "follow the career" of its subject, but instead opts for exploring the various dilemmas about art, capitalism and ownership that his work throws up; old issues around turning rebellion into money are given an extra fillip by the technical illegality of the painting in question.

Though he doesn't name him, let alone delve into his early life, Ellsworth-Jones appears implicitly to support the best-known speculation as to Banksy's identity  the Daily Mail's unmasking of him as Robin Gunningham, educated at the very exclusive Bristol Cathedral School. This is more than supported by the fact that he started out with the tag "Robin Banx" before plumping for the less obvious "Banksy", by the first-hand accounts of "one of those oddly classless people, like those you meet in the music industry", not to mention the way his stencils and prints already fit a certain earnest/ironic Bristol stoner profile, with their easy giggles and a somewhat defeated, half-hearted sticking-it-to-the-man.

Whether he is indeed a Clifton public schoolboy or an orphaned ex-docker from Knowle West, there's a productively disjointed relationship between writer and subject in The Man Behind the Wall, such as when the author describes some particularly naughty Banksy action, and recalls the punishment he would have got for it in prep school. One of Ellsworth-Jones's strengths is that he never attempts to sound hip, or even to sound as if he knew much about street art or graffiti before embarking on the biography. There are no interviews and no direct contact with the artist. As a result, Ellsworth-Jones's journeys between the seemingly inimical worlds of art and graffiti register the insular codes and rules of both better than an insider might have done.

Initially, as he follows the young Banksy through Bristol's particularly well-developed and intricate graffiti subculture, Ellsworth-Jones stoutly regards most graffiti and tagging as mere vandalism, but slowly he starts to appreciate the more baroque Bristolian examples. He outlines how Banksy both rejected and seemed to want to be respected by his graffiti elders. Early "straight" graffiti was abandoned for stencilling and clear, legible, instantly understandable sloganeering. Ellsworth-Jones evidently enjoys the fact that they want to communicate some palpable point, some pleasure in figurative form, in a subculture that prefers deliberate illegibility. Graffiti is based around putting up your private logo on the most physically hazardous place, thus displaying your skills and commitment to the extremely small group that will recognise this work as yours. Despite the amount of work that, on this account, Banksy has done for them  putting on exhibitions, renting out "licensed graffiti areas" in Bristol and London  "proper" graffiti artists apparently refuse to accept him as one of their own.

This is especially clear in the sections on Banksy's inadvertent rivalry with the older graffiti artist Robbo, one of whose London works he "improved", eliciting a still-ongoing row, which Banksy has evidently tried unsuccessfully to de-escalate. By the Regent's Canal, for instance, Banksy makes a worthy if slightly trite point by painting "I DON'T BELIEVE IN GLOBAL WARMING" just above the rising waterline. "Team Robbo" then doctors that into "I DON'T BELIEVE IN WAR", adding alongside "TOO LATE FOR THAT SONNY". It's a beautiful encapsulation of Banksy's problem, wanting to communicate at the same time as wanting to be liked by a group who place communication to the public exceptionally low on their list of priorities. Although the author doesn't make it explicit, it sounds a lot like Banksy's later encounters with the art world. Artworks authenticated by Pest Control, Banksy's own authorisation company, elicit huge crowds and large sums of money and get collected by Hollywood actors. He's a populist, loved by the public but sniffed at by the art establishment, a paranoid cousin to Jack Vettriano or Beryl Cook. This critical disdain is, apparently, due to his lack of theory, to the fact that his works can be instantly understood and enjoyed without any prior or accompanying contextualisation. Caught between two equally elitist, self-enclosed (though far from equally monied) cultures, Banksy becomes quite sympathetic in his evident desire to make serious political points and have them understood by passersby. Yet the groan-inducing obviousness and mawkish sentimentality of so much of it (rioter throwing flowers! Small girl with bomb!) is of little concern to Ellsworth-Jones.

That aesthetics is not the book's province is obvious; what makes it intriguing is a relentless following of the money, and the exploration of the tortured interface between art and commerce; Pest Control refuses to authenticate obviously genuine Banksy pieces in public places, out of fear that they'll be severed from their original locations and sold on the art market. Banksy's works and public statements now comment on this ambiguous status so often that they're almost self-referential enough to please his detractors.

 Owen Hatherley's A New Kind of Bleak: Journeys Through Urban Britain is published by Verso.

When Robert Clarke first caught sight of the then unknown Banksy in a New York flophouse in 1995, it was like one of those revelatory occasions in Hollywood biblical epics when the shadow of the saviour, whose face we are not permitted to glimpse, falls onto the ungodly. "Lo and behold," says the quivering Clarke, "he was framed in the office door and a radiant light was coming off him."

"No, no really!" he adds, but the disclaimer doesn't dispel the religiosity of the encounter. Clarke rises and follows this nondescript fellow from Bristol, who dematerialises so mysteriously and leaves behind him only prophetic daubs on the sides of buildings  anti-capitalist slogans, stencilled caricatures of greedy corporate rats, the Mona Lisa wielding a bazooka and Queen Victoria being orally pleasured by a lesbian attendant. The first writing on the wall, inscribed by a bodiless hand at Belshazzar's feast, announced the imminent fall of a city. Banksy too, for his disciple Clarke, is prophetic or "prescient", foretelling an apocalyptic future that "manifests through the walls".

Will Ellsworth-Jones is a less worshipful follower, but even he attributes a "redemptive power" to Banksy: teaching the low-born and oppressed how to assert themselves with cans of spray paint, he gives them a sense of what therapists call self-worth. Ellsworth-Jones interviews a Banksy wannabe who "explains the call of graffiti in an almost messianic way". The apostolic Clarke is content to scamper along behind Banksy, but Ellsworth-Jones's sacred narrative takes a sinister turn when Banksy sells his soul to an agent called Steve Lazarides. Ellsworth-Jones explains their association, now ended, "in biblical terms". Like Satan offering Christ the kingdoms of this world, "Lazarides took Banksy up to the mountain top, tempted him with  fame, money, success", and even threw in the alluring incentive of Angelina Jolie, a customer and a devout fan.

Banksy, who sends the credulous Clarke off to march against global commerce and the wars it foments, is shown by Ellsworth-Jones to be as capitalistic as his hero Damien Hirst. Banksy once wondered whether an artist should make money from work that was intended to draw attention to world poverty, and solved the problem by calling it ironic. He was wearing what Clarke calls his "invisibility cloak" at the time, but I can imagine lips curling in a devilish smirk as he contemplated the credulity of those who pay a premium for ephemera that were meant to mock the notion that art can be valued, traded, treated as wealth.

Clarke has the advantage of a casual acquaintance with Banksy. Together they tramp through the mud at Glastonbury, go skateboarding in Manhattan, invade London Zoo under cover of dark. But Banksy takes care to say nothing of significance and to do nothing memorable. Mostly he appears to Clarke in dreams, like Christ after his disappearance from the tomb: in one woozy reverie Banksy "writes an oath with his finger on the sacred ancestral stones" of a monument like Stonehenge. Clarke is forever regressing from the urban jungle to the rustic homeland of Merrie England, so it's good to be reminded by Ellsworth-Jones that Banksy doesn't share this new-age nostalgia. His own reconstruction of the circle of boulders on Salisbury Plain is called Boghenge, and consists of portable toilets arranged in a formation that has no astrological puzzle behind it.

Using aliases and accomplices while he remains out of sight, Banksy is a control freak. Ellsworth-Jones demystifies him by identifying the intermediaries who work on his behalf  the PR firm that publicises him by misleading the media, the organisation that authenticates his work and exposes fakes. Ellsworth-Jones also exposes his image as something of a fallacy. Clarke subscribes to the myth of Banksy's working-class roots; Ellsworth-Jones has ascertained that he attended Bristol Cathedral Choir School, which makes him a faux-prole. Banksy's first name, according to Clarke, is Robin  hardly an appellation favoured by families on council estates.

Breaking the promise of its subtitle, Ellsworth-Jones's book catches no glimpse of the man behind the wall. All he can do is contribute to his mystique. Having never been photographed  or only from behind, by Clarke's mum  Banksy has a thousand faces. He is sometimes described as a latter-day Robin Hood, though it's not clear that he's redistributing wealth as the sylvan bandit did; he is more aptly likened to the elusive Scarlet Pimpernel. But as in the case of Father Christmas, the legend, as Ellsworth-Jones says, is "better than the real thing".

Ellsworth-Jones writes perceptively about the "ethical dilemmas" created by Banksy's marketing techniques, yet still communicates the excitement of a "treasure hunt" for traces of his work in the scruffier purlieus of London. Clarke can't compete. His most eloquent tribute to Banksy's work is to call it "cool-as-fuck". Whatever you think of the aerosol guerrilla, he deserves better than that.