Skin is an up and down ride. Having just finished James Doss' certifiably worst novel, The Widow's Revenge, I cut Hayder a lot of slack. But ultimately, this is not a satisfying novel. Hayder has written three stellar mysteries -- Birdman, The Treatment, and The Devil of Nanking. Her latest three books have been strained -- trying too hard to gross us out and tipping moral centers a bit past acceptable.
For starters, no one will be able to follow this book if they haven't read Ritual (also not very good, though). There is simply too much you have to accept as given without the events of the previous novel. Secondly, even with the previous books to introduce you to the detectives (Caffrey in three, Markey in one), it's very hard to accept them as "the good guys." They don't draw much sympathy, and there are enough real good guys around (Markey's colleagues, Caffrey's pathologist) that we wonder why we should care about these two.
Next, one subplot is so revolting that it cannot come to a moral or ethical resolution (and, tellingly, it remains unresolved at the end). Suffice it to say that watching Markey spend most of the novel trying to decide what to do about a dead body in her car's trunk is neither amusing nor terribly interesting. Eventually, the situation develops to a point where we have some sympathy for the detective, but even so the problem remains insoluble and Markey's personal dilemma doesn't feel likely, nor do we identify with her supposedly hard choices.
Finally, there IS no serial killer. Since the other reviews have provided what I consider spoilers, I won't tread too carefully here. While it is true that the killer is collecting skin from victims, the killings themselves are not motivated by this but by the need to cover up his crimes. The killer himself, as another reviewer has mentioned, is something of a letdown, not because he isn't very interesting (I very much appreciate writers like Carol O'Connell, Val McDairmid and Elizabeth George for their insistence that serial killers are not, at bottom, very interesting people), but because finding and apprehending him is such a no-brainer.
And tellingly, the last murder makes absolutely no sense, because it doesn't protect his identity (the victim knows nothing about his crimes) and he doesn't violate the corpse. The victim is actually killed by Hayder in a bold-faced act of deus ex machina. Which she then reverses -- the cheapest of fictional ploys, in my view.
Hayder may have spent her capital in three excellent books. All three depend for their effect on a horrific and persuasive look at humankind at their worst. That's not much of an achievement; what made the books memorable was that she did this from a moral perspective that was completely free of sensationalism and titillation. Things went off the tracks with Pig Island, which kept the horror, lost the objectivity, and wallowed a bit in perversity. There was a lesson to learn from Thomas Harris' rapid degeneration into his own nastiness, and Hayder missed it. Ritual and Skin are headed in the right direction, but what appears to be left, now that the dust is settling, is neither compelling nor illuminating.
I'll probably read the next book in the Ritual/Skin trilogy, just to see what happens to Markey. But if that book leaves me hanging, I'll let go.