The Thursday Murder Club : (The Thursday Murder Club 1) by Richard Osman

In a peaceful retirement village, four unlikely friends meet up once a week to investigate unsolved murders.

But when a brutal killing takes place on their very doorstep, the Thursday Murder Club find themselves in the middle of their first live case. Elizabeth, Joyce, Ibrahim and Ron might be pushing eighty but they still have a few tricks up their sleeves. Can our unorthodox but brilliant gang catch the killer before it's too late?