Career of Evil kicks off (sorry) with a woman’s severed leg. It is delivered in a package to Robin Ellacott, an amiable and efficient young woman who is the new-ish assistant of private detective Cormoran Strike. You’d think it would be difficult to work out who on earth would want to send you this kind of gift. But Strike can immediately think of four people in his past who could be responsible. That is a lot of people to know who would send you a severed limb. But he is that sort of man.

Welcome to part three of the story of JK Rowling as crime writer. It’s a pretty good story in itself: bestselling writer attempts to reinvent herself under the radar and is outed almost instantly. Published in 2013, The Cuckoo’s Calling by “Robert Galbraith, a former plainclothes Royal Military police investigator who had left in 2003 to work in the civilian security industry”, was suspiciously accomplished for a debut novel and sold an estimated 1,500 copies. But Rowling’s cover was blown within weeks. Sales soared by 4,000%.

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Two years later and The Cuckoo’s Calling and The Silkworm have sold over 1.5m copies and the series is being developed for BBC1. So the stakes are high for book three. Can “Robert” (whom JK Rowling calls “my friend” on Twitter) do it again? And yet in another sense could there be any less pressure on the author? The first two books were universally well received. And it’s not exactly as if the storytelling skills of “my friend” are in doubt.

On the other hand, the Cormoran Strike novels have been described by even hardened crime fans as “macabre”. One online review of The Silkworm complains of “excessive references to Strike’s prosthesis”. Strike lost part of his right leg in an explosion while on active service in Afghanistan. (The prosthesis, removed for the – thankfully infrequent – sex scenes, is a big part of his character.) The prosthesis-averse are definitely not the target market for this book, which is all about acrotomophilia: “A paraphilia in which sexual gratification is derived from fantasies or acts involving an amputee.”

This is not quite Boxing Helena (the much-slated film starring Sherilyn Fenn as a woman amputee confined to a box by Julian Sands as a psychopathic surgeon), but there is a fair amount of limb removal as sort of perverse retribution, with Robin – Strike’s assistant – being targeted as a way to get to Strike. There is plenty to dismay the squeamish, not least the chillingly graphic passages that depict the killer’s inner world. Suffice to say, he makes Voldemort look like the Dalai Lama.

The killer delights in his cunning disguises and vast collection of knives and, er, fleshy trophies

Despite the use of the third person throughout, there is a clever interweaving of several perspectives, with the killer popping up (unidentified, obviously) throughout. He’s not much fun to be with, living with someone he calls “It” (his abused partner, we presume) and delighting in his cunning disguises (woolly hats and coats worn inside out) and his vast collection of knives and, er, fleshy trophies.

Much of the story is told with a focus on Robin, however. She is keen to make her way as a private investigator herself and wants to impress Strike, for whom she (inadvisedly) carries a small torch. And the attraction is mutual, of course. Robin’s not afraid of much – despite a major scare in her past. But she does have severe anxiety about her forthcoming wedding to weedy Matthew. Not so her stalker. He doesn’t think Robin needs to worry about getting married at all because he’s planning she won’t make it that far.

If there was a critique of the two previous Strike novels, it’s that Galbraith (yes, I will play along) privileges character over plot. This continues in the same vein but here it feels like a virtue: you care as much about Strike and Robin’s relationship as you care about the crime. A lot has gone into this book. Rowling (I’m giving up now) writes in the acknowledgements: “I can’t remember ever enjoying writing a novel more than Career of Evil.” For a writer like this that is a big “ever”. She has said elsewhere that this book required more planning than anything else she has written and that it gave her nightmares – “which has never happened before”. It gave me the heebie jeebies too. And as such I would only recommend it to serious crime fans: a page-turner with legs that will keep you up all night (sorry).

Career of Evil is published by Sphere (£20). Click here to order it for £16