In 1975, Emile Ajar's second novel, La Vie devant soi, was a French literary sensation. The fictionalised memoir of an Arab boy growing up in a Parisian suburb, packed with extraordinary slang, aggressive jokes and almost unbelievable characters, the book was lathered with praise by critics, eventually wining the Goncourt, the French equivalent of the Booker. It went on to become the bestselling French novel of the 20th century. There was only one problem: Ajar was actually Roman Gary, already a bestselling French author (and previous winner of the Goncourt, which is supposed to be awarded to any particular writer only once), who had reinvented himself to outwit the literary establishment and win a new readership.

David Bellos doesn't appear to be a huge fan of Gary's work  he describes at least one of his books as "unreadable" and others as "middlebrow"  but he's cleverly used his life to investigate the connections between a writer's fiction and his autobiography, and as an excuse for some very funny digs at literary fame, fortune and fashion.

Bellos's magnificent biography of Georges Perec, published in 1993, was an elegant portrait of a writer who claimed to have no imagination, led a fairly humdrum existence and was almost painfully honest about his own shortcomings, but wrote the most fabulous books. Gary may not have been nearly such a good writer as Perec, but he led a far more exciting life, flying planes in the war, marrying a movie star and screwing a different teenage prostitute almost every day. Even so, he felt the need to lie about his own exploits; Bellos describes how Gary had already invented himself several times before he created Emile Ajar, changing his own name, fibbing about his past and publishing some thoroughly unreliable memoirs.

Roman Kacew was born in 1914 in Wilno, now in Lithuania, although part of Poland for most of Kacew's youth. His father soon scarpered, starting another family, and his mother took her young son to France, where he went to school and then joined the air force, failing his final exams through the simple error of being Jewish.

Once the war started, such flaws ceased to matter. Gary fled to England via Morocco, joined Charles de Gaulle's Free French and flew in an RAF squadron packed with French refugees. At the end of the war, only five of his original comrades were still alive.

By then, he'd already published his first novel and married his first wife, Lesley Blanch, an English writer who offered him access to high society. Even more importantly, she didn't mind with whom he had affairs. During the rest of the 40s and the 50s, Gary worked for the French diplomatic service, taking him and Blanch to Bulgaria, Switzerland, New York and LA, where he was consul general, rubbing shoulders with the movie world. He eventually left Blanch and married Jean Seberg, star of Bonjour Tristesse and Breathless.

Meanwhile, Gary was writing novels in both French and English, often translating himself from one to the other, inventing the names of his translators or, even more oddly, paying someone to translate his work from English to French, then rewriting it himself. His books were prizewinning bestsellers. Many were filmed. Gary become a literary celebrity, regularly pontificating in newspapers and on TV.

But literary fame is fleeting; critics and readers need a constant supply of new blood. At the beginning of his career, Gary had been young, beautiful and unknown; by the 70s, he was old, wrinkled and familiar. He may have been respected, but no one wanted his books any more. So he had to create a new identity for himself, a younger man more in tune with the times. Enter Emile Ajar.

The success of La Vie devant soi turned Ajar into a major literary star, who could no longer be hidden behind veils of secrecy. Gary asked his cousin's son, Paul Pavlowitch, to impersonate the writer in interviews, but made the mistake of allowing a photograph to be distributed too. When Pavlowitch was recognised and journalists made the connection, Gary first issued statements promising that he was not Ajar and then dashed off a book, Pseudo, written in six weeks and published under Ajar's name, in which Pavlowitch acknowledged his own authorship while admitting that he was quite mad.

The critics were satisfied. Readers lapped up the books. Pavlowitch got the fame and Gary was widely pitied for being less talented than his young cousin. He had painted himself into a tragic corner; as Bellos says, "His best work, his real work, was now, by his own action, no longer his."

Pseudo was finally published in English for the first time earlier this year, nicely retitled Hocus Bogus (Yale, £16.99). In Bellos's very free translation, the novel is pun-packed, exhaustingly energetic and a lot of fun, but the narrative simply isn't nearly as interesting as the circumstances of its creation.

As for Gary himself, the effort of creating Ajar seemed to sap something within him. He published a couple more novels, one under his name and the other under Ajar's, then shot himself in the head. Like a true celebrity, he left a suicide note addressed "For the Press."

Josh Lacey's Three Diamonds and a Donkey is published by Marion Lloyd.