In which Agatha Christie novel does Hercule Poirot die?

Hercule Poirot dies in Curtain: Poirot's Last Case, Agatha Christie's final Poirot novel. The story brings the Belgian detective back to Styles Court, the country house where his first published case, The Mysterious Affair at Styles, took place.

In the novel, Poirot is elderly, seriously ill and confined to a wheelchair. He asks his old friend Captain Arthur Hastings to help investigate a series of events connected with a hidden manipulator known as X. Poirot eventually identifies X as Stephen Norton, whose power of suggestion drives other people toward murder without leaving him directly liable.

Poirot dies in his sleep, apparently from a heart attack, before Hastings understands the full solution. A letter delivered after Poirot's death explains the case and reveals that Poirot deliberately arranged events to stop Norton. Christie wrote Curtain during the Second World War but held it back for publication much later. It appeared in 1975, shortly before Christie's own death, and is the only original Poirot novel in which the detective dies.