Sleepyhead (Tom Thorne Novels) - Tapa dura

It's rare for a young woman to die from a stroke and when three such deaths occur in short order it starts to look like an epidemic. Then a sharp pathologist notices traces of benzodiazepine in one of the victim's blood samples and just traceable damage to the ligaments in her neck, and their cause of death is changed from 'natural' to murder.

The police aren't making much progress in their hunt for the killer until he appears to make a mistake: Alison Willetts is found alive and D.I. Tom Thorne believes the murderer has made a mistake, which ought to allow them to get on his tracks. But it was the others who were his mistakes: he doesn't want to take life, he just wants to put people into a state where they cannot move, cannot talk, cannot do anything but think.

When Thorne, helped by the neurologist looking after Alison, starts to realise what he is up against he knows the case is not going to be solved by normal methods - before he can find out who did it he has to understand why he's doing it.