I'm enjoying The Thursday Murder Club, by Richard Osman. It's a huge seller here in the UK, and in fact the book has come to the attention of Steven Spielberg, who has acquired the film rights.
For those who don't know, Richard is a very popular TV quizmaster and creator of successful TV quizzes. The book is about a group of residents at a retirement home, who get together to investigate old murder cases, to pass the time. Then a murder happens close to home, and they are drawn into the police investigation. It's a bit like a comedy version of Agatha Christie.