There are some things, like clearing your throat or falling in love, that you can only do for yourself. Even Prince Charles can't delegate dying or digesting to a valet. Writing your autobiography would seem to fall squarely in the category of the undelegatable; yet here we have an autobiography of Fidel Castro written by somebody else.

And not just any old somebody, either. Norberto Fuentes, one of Cuba's most distinguished writers, fought with Castro in the Cuban revolution but later fell out with him, narrowly escaped a death sentence and now lives in exile. His revenge is to steal his former comrade's psychological clothes, hijack his life-history and tell his story more truthfully than Castro presumably would himself. By donning the mask of his former political master, Fuentes can out-Castro him, confronting the reader with what he sees as the real rather than mythological figure. Fidel himself is a political fiction, an icon whose face adorns T-shirts; this novel masquerading as a life story claims to be the truth.

We know that autobiographies, like novels, slant and select. They are a kind of fiction in themselves. Even so, Fuentes maintains, everything in the book is historically accurate. This presumably includes his portrayal of the Cuban leader as a monstrous egoist, a man who drools over an obscenely detailed description of the French guillotine: "The blood spurting out under pressure from the approximately 40,000 jugulars and carotid and subclavian arteries cut during the course of the French Revolution in a procedure that took two-hundredths of a second from the release of the splendid blade . . ."

Yet the autobiographical technique is double-edged. Fuentes's fiction brings Castro magnificently alive, thus making what he presents as his casual brutality all the more repellent. Yet in doing so it can't help humanising the very figure it is out to discredit. Castro may emerge here as a cynical amoralist, but he is also something of a regular guy: shrewd, sardonic, finely intelligent and abrasively honest. He is not above the odd shaft of mordant wit. "In Cuba," he tells us, "a man has reached full maturity not when he takes a woman for the first time but when his mother stops examining his pinga [willy]."

There are less jocular allusions to the male genitals here, too. Counter-revolutionary thugs, so we are told, castrated some of Fidel's comrades and stuffed their testicles in their mouths. Recounting this kind of detail suggests that Fuentes has no interest in an off-the-peg apologia for the Batista regime. He is not some redneck refugee itching to repossess his Havana casino. Castro is not denounced from a rightwing standpoint. The only standpoint in this book is his own. Since this is an "autobiography", Castro can only condemn himself out of his own mouth.

Fuentes achieves this with admirable deviousness. His strategy is to present us with a man so swept up in political drama that he appears to have precious little inner life, and thus precious little conscience. We learn all about the struggles in the Sierra Maestra, but almost nothing about the beliefs that inspired Castro to revolt. In a near 600-page account of one of the great makers of modern history, there is scarcely an idea in sight. In penning an enthralling adventure story, Fuentes manages to empty his enemy of self-reflection, and thus of conscience. What makes for good fiction, then, also makes for a damning moral judgment. It is thus that the exile achieves his long-ripening revenge.

Making history matters a lot to the book's protagonist. "I've gone to great lengths," Castro remarks, "to ensure that my personal history is also the history of my country." Castro invents modern Cuba from scratch, rather as one might write a novel. In doing so, he "goes about the task of inventing my own monumentality". What is most real about this improbable character, in other words, is that he knows he is a fiction. But he can at least become his own author, creating his own history rather than being a function of the history of the United States.

Revolutions, as Marx recognised, are inherently theatrical events, both more and less real than everyday life. The Cuban revolution, Castro comments here, was "a miracle of the imagination". In all such mighty upheavals, fact and fiction become hard to tell apart, just as they are in this book. It is part of Fuentes's achievement to make us more conscious of these ironies. Yet there is something disturbing as well as revealing about this blow-by-blow life history. Why invest so much energy in a portrait of your persecutor? How can this avoid paying him homage in the very act of cutting him down to size?

There is something curiously obsessive about Fuentes's fascination with Fidel. Stealing someone else's selfhood is a wickedly effective way of getting even with them; yet wanting to become someone else suggests admiration as much as antagonism. For all his imaginative ventriloquism, it is hard to feel that Fuentes is aware of these ambiguities, let alone that he has resolved them.

Terry Eagleton's The Task of the Critic is published by Verso.