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. 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 . . . 

This was the longest book in the series by far. I listened to it via audiobook and it was 32 hours long. I did feel like this book did drag on a bit, but the characters were interesting. I love seeing how Cormoran and Robin keep growing and changing as well as see their friendship and relationship change. I like the mystery aspect of this series because I can never guess who the killer is, but as they are revealed and how it came to be, it all clicks together like a puzzle.