‘Have you got over your father yet?’ the 26-year-old David Cornwell was asked by MI5’s head of personnel when he joined the agency in the spring of 1958. And the answer, more than half a century later, has to be ‘no’. We knew of his conman father Ronnie’s cartoonish presence in Cornwell’s life, but never the extent to which he has dominated his very being.

After leaving Lincoln College, Oxford, Cornwell taught for a couple of years at Eton, where he disliked the ‘Herrenvolk doctrine’ expounded in what he called the ‘spiritual home of the English upper classes’. So he sought a return to the secret world that he had glimpsed as a gap-year student in Bern, after leaving Sherborne and before going up to Oxford.

While in Switzerland he was approached by a British diplomat and asked to keep an eye on fellow students — a practice he maintained at Oxford, where he developed a more formal liaison with MI5. Although a member of the exclusive Gridiron Club, he was happy to adopt a left-wing persona, join the university Communist club, and monitor potential subversives.

The strain of this double life proved intolerable. During his second year at Oxford he experienced what Ann Sharp, soon to be his wife, later called a ‘mini-breakdown’, and, while he may have struggled with the compromises he made in putting love of country before that of friends, the under-lying cause was his unresolved relationship with his father, who had just gone spectacularly bankrupt (with liabilities of £1.35 million, or more than £33 million today) and whose roguish antics enliven this book, sometimes threatening to upstage its subject.

Since Ronnie’s death in 1975, Cornwell (I follow Sisman’s nomenclature) has dithered, uncharacteristically, over his memory. He has put his father into several novels (the most autobiographical being A Perfect Spy), toyed with a play about him, spent £10,000 on private detectives to probe his background, and has several times started, and scrapped, a proposed memoir, which he now says will be published next year. This has been interpreted as a sign of his disapproval of Sisman. But, although the association between biographer and subject was often tense, it survived; and this book is testament to Sisman’s skill and perseverance.

The Ronnie stories are manifold — the bankruptcies, the extravagant living, the telephone calls that Cornwell fielded from all over the world, relating how his father’s latest scam had landed him in jail. Ronnie seduced women in his son’s name. And he had the temerity to sue him for libel over his portrayal as Aldo Cassidy’s father in The Naive and Sentimental Lover. Tellingly, while at prep school during the war, Cornwell invented a secret service career for his father to explain why he hadn’t been called up. No wonder he claimed to have rejoiced at his demise.

Despite his zeal, Sisman occasionally admits that a detail may be ‘apochryphal’, while Cornwell himself agitates over ‘false memories’. Was he really touched up by his father? And did he rebuff a pass from W.H. Auden (who told him, ‘Well, it’s nice to be fancied, isn’t it?’) — a story echoed in Smiley’s People?

One begins to understand the atmosphere of uncertainty and deception which plagued Cornwell’s early years and provided the material for his life’s work. He himself noted the relief he felt at ‘coming inside’ when, after university and teaching at Eton, he returned to the ‘priesthood’ of the security services.

Then there’s the pseudonym. John le Carré wasn’t Cornwell’s first choice. Living uncomfortably in Bonn after transferring from MI5 to MI6, he wrote his first novel, Call for the Dead, which he submitted under the name of ‘Jean Sanglas’, first to Collins, who rejected it, and then, at the suggestion of fellow thriller-writer and MI5 colleague John Bingham, to Gollancz. The publisher recommended a more macho moniker, such as ‘Chuck Smith’. Can this be true, or is it again questionable? Anyway John le Carré emerged, adding another layer to the veil between the reality and the man.

Call for the Dead went well, allowing Cornwell to joke about having to write ‘brilliant, untidy letters for future biographers’. Before long he was drawing on his MI6 experiences in Germany for The Looking Glass War and The Spy Who Came in from the Cold, which critics praised for their realistic take on modern espionage — in contrast to James Bond. The latter novel was particularly successful after it was filmed with Richard Burton in the lead role of Alec Leamas, and Cornwell became a rich man.

He moved on to his acknowledged masterpiece, generally known as the Smiley quartet, beginning with Tinker, Tailor, Soldier, Spy, which explored the enduring themes of duplicity and moral culpability through stories about double agents. The secret, remarked Smiley, was not to offer the Crown Jewels and get chickenfeed in return. Cornwell created an extra-ordinary world, centred on the Circus and its battle against Karla, as well as contributing a new, if fanciful, vocabulary of moles and lamplighters to the English language.

With his excellent grasp of the wider history, Sisman is good at anchoring Cornwell in this shadowy environment, as he guides his readers through the models for various characters (Smiley draws on Bingham and Vivian Green, Cornwell’s tutor and mentor at Lincoln).

Cornwell’s steady progress was interrupted when he met James Kennaway, a charismatic writer, whose wife Susan became his lover. This led to a torrid three-way relationship which, to the disgust of his publishers who wanted another spy thriller, he later represented in fiction in his flawed but entertaining ‘hippy’ novel The Naive and Sentimental Lover. Drawing on his admiration for German culture, the title pointed to Friedrich Schiller’s distinction between direct and reflective poetry, allowing Cornwell to examine a favourite topic — the rapprochement between freedom and society.

The affair brought an end to his troubled marriage to Ann. After ‘six months’ madness’ of sexual abandon, he met Jane Eustace, who worked in publishing and introduced him to new outlets in Britain and the United States. A gentle soul and willing helpmeet, she become his second wife.

While Ronnie’s death provided one natural break in Cornwell’s life, another was the protracted collapse of communism in the late 1980s. Deprived of Cold War subject matter, he began to write about other places and conflicts, from Russia itself in The Russia House to Africa in The Constant Gardener, his attack on the machinations of the global pharmaceutical industry. Slightly earlier, he published The Little Drummer Girl, a book inspired by his half-sister Charlotte, which took him to the Middle East, where he found his natural pro-Israeli inclinations gave way to a sympathy for displaced Palestinians.

The common theme of Cornwell’s later work is his fury at what he regards as the cynicism of the British establishment. Admitting to charges of alterszorn, the rage of age, he is particularly exercised by the unholy alliance between government and corporate power. Some critics have attacked him for becoming long-winded and polemical. But most have given him the benefit of the doubt, at times comparing his ambition to Balzac’s, and suggesting his output is worthy of the Man Booker prize.

Sisman brings admirable clarity to what could have been a meander in a wilderness of mirrors. He explains the plots and business backgrounds of the various novels. He introduces friends, ranging from Alan Clark to Stephen Fry. But there are places he doesn’t go, such as Cornwell’s operational record in MI6, as well as some personal relationships, including his affair with an anonymous woman in Bonn.

Cornwell’s latest book, A Delicate Truth (2013), topped the bestseller list 50 years after the publication of The Spy Who Came in From the Cold. As he enters his 85th year, why does he continue to bother? Well, he’s a writer, still exploring the delicate faultlines between freedom and society, and still possessed by demons, one of which, for all his professed loathing, is a nagging residue of admiration for his father.

How else should we read a passage which he excised from The Honourable Schoolboy, the book he was working on when Ronnie died? Here the journalist-turned-spy Jerry Westerby comforts himself with the thought that his own wayward father’s life was not a failure, ‘but a statement of personal freedom to strive, to love, even the freedom if necessary to fail….’ As an author, Cornwell — let’s say le Carré — remains one of our greatest defenders of such basic liberties in a complex and morally ambiguous world.