The Red Room by Nicci French

Kit Quinn is a young woman who inhabits dangerous worlds: crime scenes, interrogation scenes, hospitals for the criminally insane. Horribly wounded in an attack by a suspect, she must return to the site of her worst fears. She is asked by the police to advise them on a simple murder inquiry. A young runaway has been found killed by a canal and the chief suspect is the man who wounded Kit. But Kit refuses to accept appearances and her obsessive search for the truth draws her into an underworld of the missing and the unloved which puts her at terrible risk.