Enric Marco has had a remarkable life. A prominent Catalan union activist, a brave resistance fighter in the Spanish Civil War, a charismatic Nazi concentration camp survivor, and more. In January 2005 he addressed the Spanish parliament to mark the 70th anniversary of the liberation of Auschwitz. He is, everyone agrees, an extraordinary man. Heroic, almost.

The thing is, his extraordinary, heroic biography is at least partly a lie. But which parts?

In The Impostor, the novelist Javier Cercas seeks to disentangle Marco’s lies from those small provable truths supporting them. Cercas is reluctant at first (troubled by Primo Levi’s ‘to understand is almost to justify’); but Marco himself is a surprisingly willing participant in the investigatory process, granting Cercas multiple lengthy interviews. Does Marco’s unquenchable desire to be the centre of attention at all times simply trump his fear of being exposed as the serial liar that he is?

All Marco’s lies fit into a framework of historical truth, and the precise historical setting of Marco’s most audacious ones is significant: a country only recently out of civil war, struggling with the compromises of ‘historical memory’. Had our man been one of the many who had said ‘yes’, who had capitulated and kept quiet, or had he been a resister, a hero who’d stood up to power and said ‘no’? Marco understood that ‘he who controls the past, controls the present and the future’, so he set about furnishing himself with an optimal past.

Besides being a piece of nifty journalistic detective work, Cercas’s book is an insightful psychological study, an attempt to explain what kind of person would be driven to behave as Marco did. Is it narcissism? Can it really be as simple as that? The Marco we meet is self-aggrandising, self-justifying, a ‘mediopath’ obsessed with the limelight, a creature of insane egotism, opportunism, untrammelled imagination and a need to be loved. It’s hard not to wonder, will this book’s blast of cold reality kill him?

And how bad was his imposture, after all? Aren’t there justifiable ‘good’ lies as well as bad ones? The question is a pressing one for a novelist, of course, and Cercas uses his challenging of Marco’s fabrications to examine his own: the Quixote-like Marco takes fiction and tries to make it reality, while the novelist, except in this particular fictionless novel, does the opposite. (There’s a third readily acknowledged imposture at play here, too: the words in the English Impostor are not Cercas’s own, but those of a translator creating a highly convincing fake-Cercas voice. In this case it’s perpetrated by Frank Wynne, one of our finest Spanish-to-English impostors.)

Of course, ‘the novelist’s deception is consensual and yours was not,’ as Cercas puts it in a conversation with Marco near the end of the book. Though this one — please note — is an imaginary conversation. It’s all a little tricky, this true story about a fiction, this exposing of real-life Marco’s fictional life, a history whose every starting ‘fact’ is under threat (did Marco have two wives or three?). But the resulting study is both convincing and compelling.