Troubled Blood (Cormoran Strike #5) by Robert Galbraith

Private Detective Cormoran Strike is visiting his family in Cornwall when he is approached by a woman asking for help finding her mother, Margot Bamborough - who went missing in mysterious circumstances in 1974.

Strike has never tackled a cold case before, let alone one forty years old. But despite the slim chance of success, he is intrigued and takes it on; adding to the long list of cases that he and his partner in the agency, Robin Ellacott, are currently working on. Plus the pair are still battling their feelings for one another, while Robin is also juggling a messy divorce and unwanted male attention.

As Strike and Robin investigate Margot's disappearance, they come up against a fiendishly complex case with leads that include tarot cards, a psychopathic serial killer and witnesses who cannot all be trusted. And they learn that even cases decades old can prove to be deadly . . .

A breathtaking, labyrinthine epic, Troubled Bloodis the fifth Strike and Robin novel and the most gripping and satisfying yet.