Syd Barrett's career was over as soon as it had begun. In autumn 1964, a charming, romantic, artistically inclined youth of 16, he joined a rudimentary blues band called the Tea Set and changed their name to Pink Floyd.

In July 1967, they produced their hugely influential first album, Piper at the Gates of Dawn. By November, when they toured America with Jimi Hendrix and The Move, it was clear that something was dreadfully wrong with Barrett. He would do nothing on stage but stand open-mouthed, arms limp by his side, or de-tune his guitar and rattle the strings, or blow a referee's whistle. A favourite strategy during interviews was to adopt a catatonic stare. Back home, he would beat his girlfriend Lindsay's head on the floor.

The band brought in David Gilmour, a schoolfriend of Syd's, to help out on guitar duties. In April 1968, it was announced that Barrett had left Pink Floyd. He made two solo albums, The Madcap Laughs, and Barrett, but the songs seemed virtually dragged out of him by the supportive Gilmour and a sound engineer, in recordings that make painful listening.

In 1972, Barrett retired completely from music, from London, from the scene, and went to live in his mother's house in Cambridge. Silence and reclusion closed over him for 30-odd years. He died in 2006.

How can such a brief, tragic life justify a 440-page biography? Because Syd Barrett has always stood for more than an acid casualty. He is the damaged archangel of the sixties counterculture, a martyr to art and soul and inner space, a guy who wouldn't have any truck with commercialism.

That's the view of Sydologists worldwide, and in Rob Chapman they have found an energetic spokesman. He examines every adverse judgement on his hero and contradicts it. He doesn't believe Syd's intake of LSD was out of the ordinary. He's seen no evidence of his "madness" in documentary footage. He attributes his on-stage anarchy to "radical art statements". He wasn't even a real recluse, because he used to go to the pub now and again (by himself).

Chapman offers some genuine insights, such as the fact that all Barrett's literary heroes – Lear, Carroll, Belloc and Kenneth Grahame – lost a parent at an early age, like him. Their works are full of "disembodied identity, a dream-like sense of the self in limbo... rootlessness, restlessness, rejection, detachment, escapism".

But he takes it too far. Acres of space are employed in recalling what critics once said about the mad Victorian poet John Clare, or the Jesuit Gerald Manley Hopkins, and trying to fit their judgements to Syd's whimsical nonsense. Opaque lyrical moments are deemed "worthy of Dylan Thomas". Painter friends of Barrett are co-opted to say things like "I think Syd is quite Picasso-like."

In bigging-up Barrett as the rock visionary who couldn't stand all that rock star nonsense, Chapman flays others for their shortcomings. He attacks Pink Floyd for excessive clarity: "Waters, Mason, Wright and Gilmour (even sounds like a firm of chartered surveyors, doesn't it?) were governed by caution, deliberation, meticulous attention to detail... linearity. Syd was driven (and impeded, of course) by immediacy, spontaneity, unmediated response".

Unfortunately, none of this rescues Syd Barrett from being, or seeming, a mildly talented sixties art-school dilettante who couldn't stand the limelight and withdrew into a dazed, drugged, petulant melancholy. There was something about him, Chapman writes, "that brought out the eulogiser in everyone." Not everyone, Rob.

Syd Barrett: A Very Irregular Head By Rob Chapman, Faber & Faber £14.99