Curtain by Agatha Christie

Arthritic and immobilized, Poirot calls on his old friend Captain Hastings to join him at Styles to be the eyes and ears that will feed observations to Poirot's still razor sharp mind. Though aware of the criminal's identity, Poirot will not reveal it to the frustrated Hastings, and dubs the nameless personage 'X'. Already responsible for several murders, X, Poirot warns, is ready to strike again, and the partners must work swiftly to prevent imminent murder.

Poirot’s final case, a mystery which brings him and Hastings back to Styles where they first solved a crime together. The story was both anticipated and dreaded by Agatha Christie fans worldwide, many of whom still refuse to read it, as it is known to contain Poirot’s death.