On the surface, the mystery and horror genres have never entirely been on speaking terms with each other. Mystery's roots lie in rationalism, or the belief that reality is essentially logical, and truth intellectual: consider Sherlock Holmes using his powers of deduction to solve crimes, or Agatha Christie's Hercule Poirot exercising his "little grey cells" to identify a murderer. Horror - along with supernatural fiction, its close cousin - suggests that reality is much odder, and people do not always behave in rational ways. Or, as Stephen King puts it in his latest novel, Finders Keepers: "Deep below that rational part is an underground ocean … where strange creatures swim."
But for much of its history, the mystery genre has looked down its nose at what it perceives to be the poor, mad relatives obsessed with ghosts and ghouls, even though Edgar Allan Poe, the creator of the first series detective in English, wrote both horror and mystery stories; one of the earliest novels of detection, Wilkie Collins's The Moonstone, is steeped in fears of the paranormal; and Sir Arthur Conan Doyle, the creator of Sherlock Holmes, was a passionate believer in spiritualism.
The truth is that the two genres have much more in common than either might like to admit, for both mystery and supernatural fiction are curious about the impact of random forces on ordered lives. In mystery fiction, these forces take the form of a criminal - frequently a murderer - while in supernatural fiction it's a non-human entity, but the effect is still the same: suddenly the victim's assumptions about order and the nature of existence are altered forever.
Mystery and horror are closer still. Horror is at its most powerful when it deals with the body's capacity for suffering, whether physical, psychological or emotional. In that sense, Thomas Harris's The Silence of the Lambs is as much a horror novel as it is a thriller, just as King's Misery, to which Finders Keepers might function as an interesting companion volume, is as much a thriller as it is a horror novel.
King has long had a relationship with the mystery genre. He has written perceptive introductions to the work of the great noir writer Jim Thompson, produced two novels for the Hard Case Crime series, and even went so far as to make Lee Child's Jack Reacher a character in his 2009 novel Under the Dome, while frequently including thriller and crime elements in his supernatural fiction. Therefore, it wasn't a huge surprise when he made that interest explicit, and published his first non-supernatural thriller, Mr Mercedes, in 2014, to which Finders Keepers is a sequel.
The elegantly structured Mr Mercedes had some wonderful characters and any number of brilliant moments, including the opening vehicular slaughter at a job fair and a gruesome death by poisoned hamburger meat, but its nods to genre conventions - the retired policeman contemplating suicide, the killer with a mother fixation - diced with cliché, and the middle section moved a little too slowly, as though King were determined to show how willing he was to play by the rules.
But perhaps what Mr Mercedes lacked most was sufficient strangeness; there was the sense of a uniquely odd imagination being held in check by the perceived demands of the mystery genre. So it was that readers who had steered clear of King's supernatural fiction would have been pleasantly surprised by just how respectful Mr Mercedes was of the mystery tradition, while those who had followed his work for decades may have been slightly disappointed for precisely the same reason.
Reading the book was a little like going to the world's best sushi restaurant and having the chef serve you a very fine curry. The fact of the curry's existence might surprise and delight you, and eating it would be a pleasure, but a small part of you would still be wondering why it wasn't sushi.
Mr Mercedes, it seemed, was not to be an interrogation or re-imagining of the genre, but a gifted writer's tribute to it. It proved that King could write a straight crime thriller that was as good as almost any out there, but the question remained: was that what we wanted of Stephen King?
Finders Keepers offers a partial answer. It's a more idiosyncratic novel than its predecessor, and returns to a theme clearly close to King's heart, one previously explored in Misery and Lisey's Story, among other works: a toxic relationship between an author and a fan.
John Rothstein, a reclusive novelist reminiscent of the late JD Salinger, is murdered by an obsessive reader named Morris Bellamy, in part because Bellamy is unhappy with the way that Rothstein ended his famous "Runner" trilogy of novels many years earlier.
But Bellamy discovers that Rothstein had continued to write, if not publish, and sees a way to make a fortune off the previously unknown works. Unfortunately, Bellamy ends up in jail before he can put Rothstein's manuscripts to use, which is where everything starts to get interesting.
In its fascination with books, writers and readers, Finders Keepers is more recognisably a Stephen King novel than Mr Mercedes. It's also a slower burn, in part because its timescale covers more than three decades, and is so classically ordered that the ending is never really in any doubt, although that's part of its pleasure: the reader is not so much tied up with what unfolds, but how it unfolds.
Finally, the touch of the weird so absent from Mr Mercedes is present in Finders Keepers. Either King rapidly grew tired of adhering too closely to the genre's rationalist roots or, as is more likely, he was playing a long game from the start with this proposed trilogy of books. (The Suicide Prince, the final instalment, will appear next year.)
For now, Finders Keepers is a first-rate crime thriller, but it's also to King's credit that, more than 40 years into his career, he continues to experiment, and his talent, curiosity, and generous, humane spirit show no signs of flagging.
John Connolly is the author of A Song of Shadows, published by Hodder & Stoughton
Hodder & Stoughton, hbk, 370 pages, £20