The Murder of Roger Ackroyd
One of the most ingenious  of Agatha Christie's novels. Narrated by a Doctor Sheppard, who takes the place of Hastings (now living in Argentina with his wife) in assisting Poirot. A wealthy widow in the village of King's Abbott, Mrs. Ferrars, is found dead, and Dr. Sheppard suspects suicide until Roger Ackroyd, a widower who was expected to marry her, is also killed. Poirot is Sheppard's new neighbor, and is relieved to escape the boredom of the vegetable marrows he has been growing by investigating the case. Most of the suspects were Ackroyd's house guests, including Ackroyd's niece, Flora; Major Blunt, a big-game hunter romantically interested in Flora; Geoffrey Raymond, Ackroyd's secretary; Ursula Bourne, a parlormaid; and Ralph Paton, an adopted son with gambling debts. Poirot is also assisted by Sheppard's sister Caroline, a middle-aged village spinster who anticipates Miss Marple's character. Both a play (1928) and a film (1931) were made of the story, both called "Alibi".