As he lay dying in the discomfort of a hospital bed in Switzerland, Nabokov wrote a letter about “The Original of Laura, the not quite finished manuscript of a novel which I had begun writing and reworking before my illness and which was completed in my mind: I must have gone through it some 50 times and in my diurnal delirium kept reading it aloud to a small dream audience in a walled garden. My audience consisted of peacocks, pigeons, my long-dead parents, two cypresses, several young nurses crouching around, and a family doctor so old as to be almost invisible.”

Nabokov decreed that, if the book was still incomplete when he died, it should be destroyed, its secrets safe with the peacocks and crouching nurses of his imagination. But in 1977, when the celebrated author of Lolita finally succumbed to congestive bronchitis, his wife, Véra, and his only child, Dmitri, found it impossible to kill off this final novel. Nabokov’s family hoarded the 138 index cards, the last traces of a man they loved, and the final words of one of the 20th century’s master prose stylists.

Professor Brian Boyd, Nabokov’s biographer, says it was “a very human dilemma. They wanted to follow the wishes of someone they loved unreservedly, and they deeply respected his reason for wanting the manuscript burnt – his commitment to artistic perfection, so far as he could achieve it. But they also valued his work with a passion, and wanted no scrap to be lost, and wanted the hard efforts of his last two difficult years in and out of hospitals not to have come to nothing.”

It took Dmitri, now 75, a long time to open the box: “I had to traverse a stifling barrier of pain before touching the cards he had lovingly arranged.” But after studying the contents, Dmitri declared Laura to be, “An embryonic masterpiece whose pockets of genius were just beginning to pupate here and there on his ever-present index cards.” (Nabokov did not type, and wrote all his novels in pencil.) The cards, said Dmitri in a telephone interview with The Times last month, “were my father’s version of the computer, there to be moved and rearranged”.

From Dmitri’s apartment in Montreux, rumours of the curious contents of the book dripped out. Two of the cards popped up in a learned journal and Ron Rosenbaum of The New York Observer began a campaign to save the unfinished manuscript from destruction. Four years ago, Rosenbaum received a worrying e-mail from Dmitri, which seemed to suggest that he would destroy Laura before his own death, partly because of his father’s wishes, and partly because of the repellent atmosphere of “Lolitology” these days – the “lesser minds” on the internet and in parts of academia who were ascribing increasingly arcane and perverse motives to Nabokov’s creation of Lolita. “Dmitri’s Dilemma” made headlines and television debates. Scholars were in a cold sweat, wondering if Nabokov’s son would press the “delete” button.

But there was a precedent: it is well known that Nabokov tried to burn the manuscript of Lolita, after it was turned down by American publishers. Yet issued later by the Olympia Press in Paris, Lolita went on to cement Nabokov’s reputation as a great modernist, made Humbert Humbert and his nymphet infamous, and sold more than 50 million copies worldwide. This success was due to the timely intervention of Véra Nabokov, who twice saved the 12-year-old Lolita from a premature death in the Nabokovs’ incinerator when they lived in Ithaca, New York.

After Véra died, Dmitri became his father’s sole literary executor. As an opera singer and racing car driver, he lived a glamorous life and never married. Later, he became involved in translating Nabokov’s work from his native Russian, and maintaining his literary reputation with meticulous attention to detail. In photos, Dmitri’s flat shows a poster-sized image of his father, staring down upon him. In his introduction to Laura, Dmitri explains how powerful his father’s presence was.

“When the task passed to me, I did a great deal of thinking. I have said and written more than once that, to me, my parents, in a sense, had never died, but lived on looking over my shoulder in a kind of virtual limbo, able to offer a thought or counsel in order to assist me in a vital decision, were it a crucial mot juste or some more mundane concern. If it pleases an imaginative commentator to liken the case to mystical phenomena, so be it.” Guided by the ghostly advice of his parents, Dmitri decided he would publish – without being damned.

“I waited a long time because of my father’s interdiction,” said Dmitri, “but I thought he might have reversed it, at a less painful time. After I agreed to publication, and had almost finished the editing, the doctors told me I was destined for the grave myself.” Dmitri has type 2 diabetes and polyneuropathy, which causes intermittent loss of sensation in his feet. This was compounded by “a bad spell of pneumonia”, and he fell into a coma last spring. When he eventually recovered, he was “very relieved but annoyed because there was some fine-tuning of the text that I hadn’t had time to do.”

Nabokov’s exact words from beyond the grave are not detailed, but Dmitri did reveal in an e-mail to a television debate that: “My father, with a wry and fond smile, might well have contradicted himself upon seeing me in my present situation and said, ‘Well, why don’t you mix the useful with the pleasurable? That is, say or do what you like, but why not make some money on the damn thing?’ ”

“The damn thing” has been beautifully published by Penguin here, and everyone is making money. The cards are reproduced with perforated edges so readers can press them out and rearrange them, as Nabokov did. On later ones, the book disintegrates into notes. “But the finished writing crackles with alertness,” says Professor Boyd. “There are notes to himself that show haste, perhaps distraction or loss of focus, perhaps the rush of other ideas.”

The subtitle of The Original of Laura is Dying Is Fun. Thus the book, with its black covers, is both a novel-cum-jigsaw and a memento mori. Dmitri says that, “Nabokov himself expressed to me some doubt about using Dying is Fun as a title. But fun – ie, the ludic element – was always an essential part of Father’s work.”