Rabbit Hole by Mark Billingham

Alice Armitage is a police officer. Or she was.

Or perhaps she just imagines she was.

Whatever the truth is, following a debilitating bout of PTSD, self-medication with drink and drugs, and a psychotic breakdown, Alice is now a long-term patient in an acute psychiatric ward.

When one of her fellow patients is murdered, Alice becomes convinced that she has identified the killer and that she can catch them. Ignored by the police, she begins her own investigation. But when her prime suspect becomes the second victim, Alice's life begins to unravel still further as she realizes that she cannot trust anyone, least of all herself.