Troubled Blood by Robert Galbraith

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. And Robin herself is also juggling a messy divorce and unwanted male attention, as well as battling her own feelings about Strike.

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...