The Thursday Murder Club by Richard Osman

In a peaceful retirement village, four unlikely friends meet weekly in the Jigsaw Room to discuss unsolved crimes; together they call themselves The Thursday Murder Club. 

When a local developer is found dead with a mysterious photograph left next to the body, the friends suddenly find themselves in the middle of their first live case. They might be senior citizens, but they are clever than most. As the bodies begin to pile up, can our unorthodox but brilliant gang catch the killer, before it's too late?