I gotta be honest: I don't read this series for the mysteries. I read it for Robin and Strike. Still, it delivered on all fronts.
The fifth book in the series, Troubled Blood finds the detective duo tackling the cold case of a woman missing for over 40 years. At the time of her disappearance there was an infamous serial killer on the loose, and her surviving family (primarily her daughter) are desperate to know if she was one of the unidentified victims of the killer.
The book is huge, over 900 pages, and the story unfolds over the course of a year. The crafting of the mystery is superb. There are a lot of elements in this book that could be considered cliche, including an incarcerated serial killer that the detective needs to consort with, crazy occult stuff, and an unassuming yet deceptively villainous nurse. But in the hands of a master like Rowling, everything is so well developed and so well paced that it feels fresh and engaging. Her writing flows effortlessly, and there were plenty of memorable scenes. I was especially fond of Strike's interrogation of the serial killer. I thought his strategy was fairly obvious, but the pleasure was in watching the conversation develop.
In addition to the cold case, there were other active cases on the agency's books, as well as personal-life drama that kept both Robin and Strike occupied in their off hours, and otherwise prevented them from delving too much further into the notion of any romantic relationship. I will say I was hoping for more than we got in this book, which didn't really move the needle any further than the last book did. I also thought the sub-plot with Strike's biological father was predictable, and that Robin's subplot with the agency's pervy subcontractor was both unnecessarily long and kinda sexist. Robin's a robust character, and while I was very satisfied with how she finally resolved the situation, she should've acted a lot sooner. And preferably used the knife.
It's not a perfect book, or even the best book I've read, but it's still well worth the time to read all 900+ pages, not to mention the time it takes for it to become available at my local library.