Marsupials are pouched animals, mostly from Australia, that give birth to their young in an unfinished state. What we have here is a weird marsupial hybrid. It's part Australian WikiLeaks founder Julian Assange, and part Scottish novelist and ghostwriter Andrew O'Hagan. This mixed-up creature has given birth to an unfinished draft, dragged out of its pouch and published before its maturity under the wacky title The Unauthorised Autobiography. Assange hasn't really been well-served by his publisher's behaviour. It's the result of what seems to be a characteristic Assange imbroglio in which he will neither give back his £412,000 publisher's advance, nor deliver a finally approved manuscript. But the decision by Canongate's Jamie Byng to publish regardless, although understandable, has produced an unsatisfactory book.

The ghostwriter and his subject hadn't yet really gelled by the time of this draft. It's easy to see the fictionalising hand of O'Hagan in an early chapter about Assange's hippy boyhood in northern Queensland. It begins, soulfully: "For most people, childhood is a climate. In my case, it is perfectly hot and humid with nothing above us but blue sky " But a later section on the Aussie hacker's souring partnership with the journalists who were to print his leaked US secrets is much more raw. The opening reads like Assange sounding off verbatim on a bad day, in a sentence full of bile and misogyny: "Vanity in a newspaper man is like perfume on a whore: they use it to fend off a dark whiff of themselves." For by the time we reach this second half of the book, O'Hagan's mediating intelligence seems to have retreated, and the digital recorder is doing much of the work. Perhaps the ghost got weary, locked up in a chilly East Anglian winter with his monologuing subject, who is currently confined there on bail, fighting extradition on Swedish sex allegations.

The lack of a final edit does other disservices to Assange's story. The narrative stops too abruptly, before publication in the Guardian and the New York Times of the third and most important set of leaks he had acquired (the state department cables), and the subsequent legal pursuit of Assange on the sex complaints. It's padded out instead with unnecessary chunks of the cables themselves, which can be read elsewhere. The unresolved criminal allegations, inevitably, make him censor a defensive account of sex with two Swedish fans. It's all very well calling a woman "neurotic", but did he deliberately tear a condom as she alleged?

Furthermore, a nervous Canongate libel lawyer, no longer able presumably to rely on Assange as a future witness, appears to have simply chopped out chunks of detail when Assange abuses those he doesn't like. This censorship muddies what could have been a lively, if defamatory, narrative, and pointlessly withholds many of the names. I myself, for example, who clashed with Assange during the Guardian saga, and co-authored a book he didn't care for, am anonymised throughout, transparently enough, as "the news reporter". Yet Bill Keller, then editor of the New York Times and considered presumably to be libel-proof under US free-speech laws, remains relentlessly vituperated against under his own name.

A final fact-check would have removed a crop of stupid errors. It must have been a transcription mistake that turned Heather Brooke into "the 'Independent' journalist" rather than the independent journalist she is. And Oscar Wilde with his rent-boys was not "sleeping with panthers", he was feasting with panthers.

For all its drawbacks, the memoir does add some good detail to the increasingly well-trodden field of Assange studies (it's the fifth book so far). The passage in which he meets his biological father, a bohemian Sydney actor, for the first time in his 20s, is genuinely poignant: "I found myself getting sort of angry  There on shelf after shelf were the exact same books as those I had bought and read myself  If I had only known him, I might just have picked his books down from the shelf  I was forced to make myself up as I went along."

And there's a telling section in which Assange, perhaps unwittingly, reveals why he seeks out unquestioning disciples, and quarrels with so many others: "Opponents past and present have the same essential weakness about them  first they want to use you, then they want to be you, then they want to snuff you out. It's a pattern that stretches in my life from toytown feds to hacks at the Guardian  Usually it ends with these people enumerating one's personal faults, a shocking, ungrateful, unmanly effort, to be filed under despicable in my book  I've been meeting [these people] all my life."

This seems to be a cry coming from a truly threatened personality, in fear of being overwhelmed and extinguished. People have criticised Assange for being preposterously grandiose and lashing out at imagined "enemies". Perhaps they should have been kinder, for there is clearly something else at work here.

It's a shame Assange couldn't get on with the Guardian. As he has the grace occasionally to recognise in this book, people there share some of his beliefs  free speech, investigative journalism, standing up to big corporations and murderous governments, the potentially liberating quality of the internet. And his idea for WikiLeaks provided an exhilarating addition to the world's journalistic possibilities. It was a neat tool  as an uncensorable global publisher of last resort, and as an electronic outlet for leaking the new kinds of huge database the computer age is bringing into existence. But unmediated leaking on a random basis, even of gigabits of purloined documents, cannot ever revolutionise all the world's power relationships. There Assange shows, regrettably, that he is living in a fantasy world.

Behind his high-sounding talk of quantum mechanics and global conspiracies, there lies a more familiar and heartfelt cry: "If only people knew what was really going on, they'd do something about it!" One sympathises. But these very memoirs demonstrate the opposite. Nothing much happened after Assange threw back the curtain to reveal his sensationally leaked Baghdad helicopter gunship video, with US pilots mowing down Reuters employees and young children in a burst of incompetent cannon-fire. As Assange (or O'Hagan) concedes: "It vexes me when the world won't listen."

That was what forced him to accept an offer from some of the world's major newspapers to make sense of the rest of his material, publish it under the authority of their own names and grant him a share of the credibility slowly built up over 190 years of reputable reporting. Thanks to that imaginative transaction, he rocketed briefly to worldwide fame. These marsupial memoirs of his seem unlikely to increase his prospects of becoming the messiah of the information age. Maybe, sadly, even the reverse.

David Leigh is the co-author of WikiLeaks: Inside Julian Assange's War on Secrecy (Guardian Books).