The Murder of Roger Ackroyd (1926).
Hercule Poirot retires to a village near the home of a friend, Roger Ackroyd, to pursue a project to perfect vegetable marrows. Soon after, Ackroyd is murdered and Poirot must come out of retirement to solve the case.
It is one of Christie’s best known and most controversial novels. There are MANY suspects.
There have been many adaptations.