The “true crime” genre, formerly beneath the notice of the chattering classes except for the odd highbrow specimen such as Truman Capote’s In Cold Blood, has recently taken pride of place in the cultural mainstream, largely thanks to a run of glossy documentaries and podcasts that have made prurient interest in lurid murders seem respectable by applying a veneer of sociological or psychological inquiry.

Several of the genre’s practitioners have defended it very eloquently against charges of being insensitive or exploitative, although their arguments sometimes seem to me eerily reminiscent of those that Thomas De Quincey laughed out of court back in 1827 in his ironical essay “On Murder Considered as One of the Fine Arts”.

John Darnielle’s third novel, most of which is narrated by a bestselling author of true crime books called Gage Chandler, confronts the moral ambiguity of the trade head-on. Anybody who is familiar with Darnielle’s previous books, or his music – he is the frontman of the indie band the Mountain Goats – will know of his affinity for the downtrodden and the marginalised, the sort of people who would be powerless to prevent somebody else from refashioning the story of their lives or those of their dead loved ones into an authorised version that displaces the truth in the public consciousness.

Not that Chandler is not a decent fellow, ruefully aware of the parasitical nature of his work, and determined to elevate it above that of his fellow practitioners. “I try to honour the dead in my books. It’s one of the things, I hope, that sets me apart a little from my partners in true crime.”

His latest project sees him move to a crummy part of Milpitas, a suburb of San Francisco; his new home is a renovated pornography store where a horrific double murder was carried out in the 1980s, allegedly by a coven of teenage Satanists who squatted there. The soulful Chandler spends as much time communing with the aura of the place – “I feel a ghostly kinship with the lives once lived [here]” – as interviewing the townsfolk.

But then he receives a letter from somebody involved in another cold case he has written about – a teacher who chopped up two of her students in the 1960s, and ended up on Death Row when her self-defence plea was met with accusations that she practised black magic – and the novel morphs from apologia into apology as he starts to question whether he has done more harm than good. The book shape-shifts disconcertingly: what seems to be a Gonzo-ish account of Chandler’s investigation into the porn store murders transmutes into a tricksy, metafictional demonstration of the unknowability of truth.

The novel is marbled with juicy violence and often suspenseful, but Darnielle is not afraid of slowing the tempo right down to indulge in eccentric interludes such as a parody of Le Morte d’Arthur, or in extensive philosophising. Chandler’s metaphysical speculations are often eloquent and just as often grandiloquent – and Darnielle is in the lucky position of being able to take credit for the eloquence and blame the grandiloquence on the character.

When Chandler describes a radio in a photograph of a crime scene – “The top four holes are decorative rather than functional, framing a colourful pattern: red, yellow, yellow blue. Why two yellows instead of a four-colour spread? Who can say?” – Darnielle must be parodying the sort of true crime writer who strains to invest meaningless details with significance. Mustn’t he?

One of Darnielle’s finest achievements in his previous two novels was his success in evoking the texture of life as a teenager, its fleeting glory even for those who are disaffected and dysfunctional. He tries to do the same thing here but to diminished effect, because we are seeing his teenage characters distortedly and at a remove – mediated through the eyes of Chandler as he attempts to reconstruct their involvement in the murders years earlier, and then questions his ability to do so.

This shortcoming is appropriate, in a way – the theme of the novel, after all, is that it is arrogant folly for a non-fiction writer to believe he can truly get to the bottom of any human heart. In fiction, though, writers can attempt that feat, or simulate it, without betraying anybody: Darnielle has hobbled himself here by not allowing Chandler (and therefore himself) to do so, with the result that the novel has a lack of emotional impact that its other pleasures don’t quite compensate for.

Devil House is published by Scribe at £14.99. To order your copy for £12.99 call 0844 871 1514 or visit books.telegraph.co.uk