In April 1975, Simon Wiesenthal, the Holocaust survivor whose efforts to track down Nazi war criminals had earned him the title of “Nazi hunter”, wrote a letter to Albert Speer, the Nazi war criminal. Wiesenthal thanked him for a psychology book Speer had sent him, and forwarded a copy of the French edition of his own memoir. Their decade-long correspondence also includes holiday postcards and birthday wishes. It ends with a personal note from Speer’s widow Margarete on her husband’s death in 1981, telling Wiesenthal how important their friendship had been to him.

Wiesenthal’s friendship was a private echo of the extraordinarily warm international welcome that Speer received as a public intellectual after his release from Spandau prison in 1966. Speer had served as minister of armaments in wartime Nazi Germany, and was found guilty of crimes against humanity; yet when he died, he was in London to promote his new book on the BBC.

Speer’s rehabilitation was a masterpiece in duplicity. In his defence at the Nürenberg trials – and in later books and interviews – he was the only high-ranking official to take on full responsibility for the Nazi crimes; and this seeming moral clarity allowed him to credibly lie that he had not known about the extermination camps. The evidence for that would emerge only after his death, prompting Wiesenthal, among many others, to admit he had been duped. Until then, the lie allowed Speer to become an authority on the endlessly fascinating topic of Adolf Hitler’s personality and psyche.

Speer’s relationship to Hitler is at the heart of French author Jean-Noel Orengo’s You Are the Führer’s Unrequited Love, a masterfully unconventional novel about Speer’s two lives: as Hitler’s personal architect, ally and confidant; and as the world’s idealised specimen of a “good Nazi”: articulate, repentant and ultimately not as monstrous as others had been. Orengo’s book is not a biography, in that it is not concerned with exhaustivity and does not offer any previously unpublished scholarship. It is also not a novel proper: it does not rely on narrative invention to flesh out the details of pivotal moments, characters or scenes. It reads as the character study of a man who manipulated one of the most powerful men on Earth – and, after the latter’s defeat, his victors – into believing he, Speer, was exactly what they wanted him to be.

This is a mechanism of seduction, and You Are the Führer’s Unrequited Love also reads as a love story of sorts. The title is a remark an SS officer once made to Speer; no serious historian has taken it literally, but it does seem to reflect the deep, tumultuous nature of Hitler and Speer’s friendship. The book opens with their first meeting, and chronicles Speer’s lightning ascent from twentysomething stage designer to minister of armaments in less than a decade. The narration moves swiftly from archetypal scene to archetypal scene: a growing intimacy between the two men; the interplay of anxiety and avoidance; spats and reconciliations; the final parting in Hitler’s bunker just before his death. Orengo isn’t interested in speculating on what actually went on between them, so much as in the psychology underpinning it. Each had something the other craved: famously handsome, Speer offered a connection to art and beauty, the validation of the German cultural elite that Hitler envied and despised. In exchange, Hitler offered a vehicle for Speer’s boundless ambition. The story is told in a memoirist’s sparely lyrical, inquisitive voice; the facts of Speer’s life and of the war raging in the backdrop are cues for Orengo’s musings on the inner calculations and deep drives that allowed Speer to win Hitler’s confidence and, later, betray it. Was it ruthless narcissism? Crippling insecurity? Naked ambition? Are these different things?

The same question arises in the second part of Speer’s life, which Orengo describes as that of a “merry widower”. Here he focuses on Speer’s friendship with the writer Gitta Sereny, a former member of the French resistance who set out to interview him for the Times and ended up becoming the witness – albeit not a totally convinced one – to his good faith. As with Hitler, Orengo artfully shows how Speer was able to sense, and play into, Sereny’s hopes and desires, transforming into exactly the opposite of what he had been before in order to remain a success.

Sereny’s book about Speer is titled His Battle with Truth; truth is also a chief concern in Orengo’s work, itself a true story hinging on lies. That is a good definition of the opposite of a novel, and in the last chapter Orengo explains how a reflection on the political value of literary forms lies behind You Are the Führer’s Unrequited Love. Speer’s divergent tellings of his own story are, Orengo claims, history’s most significant piece of autofiction: Speer understood that “celebrity operates very much beyond good and evil”, and tinkered with facts in order to maximise his fame. Orengo sees Speer’s trajectory as emblematic of the risks of a society losing control of the boundary between invention and fact – a forewarning of the post-truth era. “When you recognise that,” Orengo muses repeatedly, almost as a refrain, “pessimism is the only wisdom.”

• Perfection by Vincenzo Latronico is published by Fitzcarraldo. You Are the Führer’s Unrequited Love by Jean-Noël Orengo, translated by David Watson, is published by Penguin Classics (£14.99). To support the Guardian, order your copy at guardianbookshop.com