I’ve been marooned in Normandy for the last nine weeks, in an isolated, semi-built house in the middle of nowhere, scanning the horizon for any sign of ferries shunting into the port at Caen. My fellow inmate compares us to Robinson Crusoe and Friday, but I feel closer to Tony Last in Evelyn Waugh’s A Handful of Dust, who is trapped for eternity in the Brazilian jungle reading the complete works of Charles Dickens to his captor, Mr Todd.

This is because I too am reading the complete works of Charles Dickens. I feel yoked to Dickens in some strange way because the day on which I was born, 9 June, was also the day on which he died. A century separates the two major events but still, we share an anniversary. Because this month is the sesquicentenary of his death, I’ve been investigating his last work, The Mystery of Edwin Drood, which, had it been completed, would have marked a major new direction in his writing. Like all of Dickens’s novels, Edwin Drood was serialised and three of the twelve planned instalments had appeared when the author collapsed following a stroke on 8 June. Three further instalments ran in the months immediately after his burial, at which point – bang in the middle of the mystery – the narrator’s voice fell silent and millions of readers were left on tenterhooks.

Edwin Drood, released from his engagement to Rosa Bud, has disappeared. Last seen in the city of Cloisterham on Christmas Eve, he was preparing to dine with his uncle John Jasper and the stranger Neville Landless. Both men have tempers and both are also in love with Rosa. Has Edwin been murdered? If so, by whom? And where on earth is his body? Typical of Dickens to leave us with an unsolvable game of Cluedo. Was it John Jasper with the necktie in the cathedral, Stony Durdles with the fibula in the graveyard or Mrs Crisparkle with the arsenic in the close?

The case of Edwin Drood turns us all into Inspector Buckets. Droodiana has been a thriving industry since 1873, when an American spiritualist called T P James published Part Second of the Mystery of Edwin Drood, claiming that Dickens had dictated the ending to him from beyond the grave. In The D Case, or the Truth About the Mystery of Edwin Drood (1989), Carlo Fruttero and Franco Lucentini gave Auguste Dupin, Sherlock Holmes and Hercule Poirot the chance to each try his hand at decoding the puzzle.

We know, however, who killed Edwin Drood because Dickens told his son, and his illustrator, and his first biographer, John Forster, that John Jasper was the murderer; he even tried to tell Queen Victoria, but she wasn’t interested. The real mystery, as A N Wilson argues in his absorbing The Mystery of Charles Dickens, is the personality of Dickens himself, a man as divided as one of those Venetian masks that have the devil on one side and an angel on the other.

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While he was writing Edwin Drood, Dickens was imbibing large quantities of opium, a drug that allows us, Thomas De Quincey explained in Confessions of an English Opium-Eater, access to a ‘second life’. Opium also allows the ‘guilty man’, De Quincey continued, to regain ‘for one night … the hopes of his youth, and hands washed pure of blood’. But at the same time as releasing the guilty man from his chains, taking opium causes further feelings of guilt. ‘In the one crime of OPIUM’, wailed Coleridge, whose life was destroyed by it, ‘what crime have I not made myself guilty of!’

Dickens was the master of guilt: Little Dorrit’s Arthur Clenham is a study in the psychopathology of the guilty man, and so is John Jasper. But it was his own guilt that increasingly haunted Dickens, and the figure of Jasper was a disguised self-portrait. According to his daughter Kate, Dickens was ‘as absorbed in the study of the criminal Jasper, as in the dark and sinister crime that has given the book its title’. That his interest was in the criminal rather than the crime is confirmed by Forster, who was told by Dickens that the novel’s second half was to be set in a prison cell where Jasper was writing his confession, as though his ‘wickedness’ had been the action of ‘some other man’.

The Jasper who murdered his nephew, in what was probably an opium-induced fugue, was therefore not the same person as the Jasper who loved his nephew. So in its completed form, The Mystery of Edwin Drood was to have been Dickens’s version of Confessions of an English Opium-Eater.

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But this was not the only De Quincey essay that Dickens had in mind while writing Edwin Drood: the entire novel is, I suggest, a homage to De Quincey. Chapter One begins with Jasper waking up in an opium den on the Ratcliffe Highway in east London in the company of ‘a Chinaman, a Lascar, and a haggard woman’. It is not irrelevant that an aspiring murderer should find himself here, of all places. Today the Ratcliffe Highway is a faceless thoroughfare but in December 1811, two months before Dickens was born, it was the scene of a notorious set of murders in which the inhabitants of two households had their throats cut by a mysterious assailant. A sailor called John Williams was arrested for the crimes but he hanged himself before being brought to trial. ‘The sensation excited by these most ferocious murders’, The Times reported, ‘has become so general, and the curiosity to see the place where they were committed so intense, that Ratcliffe Highway was rendered almost impassable by the throng of spectators.’

In 1857 De Quincey, whose interests in opium and in Williams continued throughout his life, reconstructed the Ratcliffe Highway murders in a startling postscript to his essay ‘On Murder Considered as One of the Fine Arts’. The Ratcliffe Highway, De Quincey wrote, was a throng of ‘Lascars’ and ‘Chinese’, and the ‘hellkite’ making his way through the crowds in December 1811 was no common murderer; he was an aesthete, a connoisseur and a man of taste – much like Jasper himself. De Quincey’s interest was also in the criminal rather than the crime. ‘There must be raging some great storm of passion – jealousy, ambition, vengeance, hatred – which will create a hell within him,’ he wrote of Williams. ‘And into this hell we are to look.’

In the final scene of Edwin Drood, Dickens takes Jasper back to the Ratcliffe Highway, ‘eastward and still eastward through the stale streets’, where he smokes another pipe. He is now dressed in mourning, and so the haggard woman asks who died. ‘A relative,’ Jasper replies. ‘Died of what, lovey?’ ‘Probably, Death,’ Jasper replies. The next day Dickens, too, was dead, having turned the hell within him into a fine art.