Enric Marco was one of the most famous men in Spain. As President of the “Amical de Mauthausen”, an association of Spanish survivors of the Nazi camps, he spoke eloquently about the evils of Fascism. In a speech given to the Spanish Parliament in 2005 his account of his experiences in a concentration camp had the children of deportees in the gallery in tears. This wasn’t all. He had fought on the Republican side in the Civil War, been persecuted by the Franco regime and then, as Secretary-general of the CNT (the Anarchist trade union) been influential in the transition of Spain from dictatorship to democracy. Quite a life! What a hero!

But then came exposure; he was unmasked. He had never been in a concentration camp, though he had been in Germany during the war as a volunteer worker. His anti-Fascist credentials were soon questioned. If he had lied about the concentration camp, why should he be believed about anything? Yes, he had been a magnificent and compelling speaker about the horrors of Nazi Germany and Franco’s Spain, but he was still an impostor, mocked and reviled.

Cercas probes this mysterious and extraordinary life with uncommon patience, uncommon skill and uncommon sympathy. He reminds us that he is himself a novelist and that novelists tell lies as a means of pointing to some sort of truth. The Spanish philosopher José Ortega y Grasset thought that one way of understanding an individual’s life was to see it as a novel in the making. Is this the key to understanding Enric Marco: that he made a novel of his life? Or was he just an unscrupulous liar who would say anything to be loved and famous?

Cercas is too good a novelist to find a definite answer to his quest; readers of this fascinating book, admirably translated by Frank Wynne, are invited to make their own judgement. Cercas has given us the material to do so.

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?

Its subject is Enric Marco: an actual person, now close to 100. In the 1980s, as newly democratic Spain began to recover its public memory of civil war and dictatorship, this Catalan trade unionist emerged as the charismatic spokesman for Spanish survivors of deportation to German concentration camps. In countless talks, Mr Marco brought tragic history to life, bearing witness to Nazi barbarism. Then, in 2005, a historian unmasked him as an impostor, “a compulsive, barefaced liar”. Mr Marco had gone to Germany, but as a volunteer worker, not an inmate. He fabricated his anti-Franco exploits. Yet, after exposure, the “shameless charlatan” justified his pretence as a “noble, altruistic lie” that opened younger eyes to the evils of the Holocaust. Unabashed, this “novelist of himself” continued to “gild his biography with an epic lustre”.

Mixing dogged research and testy, sparring interviews with the charming pretender, Mr Cercas scrupulously tracks Mr Marco’s big lie. As an author who juggles reality and fiction, he interrogates his own attraction to this saga of deceit: “Perhaps only an impostor could tell the story of an impostor.” As he peels away the “onion skin” around this “peerless trickster”, does the narrator also create a saintly fiction of himself as the fearless slayer of falsehoods? Mr Cercas links Mr Marco’s imposture to the “industry of memory” in post-Franco Spain, as “the entire country was reinventing itself.” He shows this contested past, and his interpretations of it, as a play of masks and myths built around “an enormous collective lie”. No heroic rebel, the conformist Mr Marco “always sides with the majority”. Indeed, his “narcissistic and kitsch” hoaxes reveal nothing less than “the true history of Spain”.