The final Hercule Poirot novel was published in 1975.
That novel is Curtain: Poirot’s Last Case, which Collins Crime Club released in the United Kingdom in September 1975. It brings Poirot and Captain Arthur Hastings together at Styles Court, the setting of Christie’s first Poirot novel, The Mysterious Affair at Styles. The return to Styles gives Poirot’s career a deliberate sense of closure.
Christie actually wrote Curtain much earlier, around the Second World War, and kept it unpublished for decades. She stored it in a bank vault, intending it to be released later so her daughter would have an inheritance if Christie died during wartime. The novel was finally published only a few months before Christie’s death in January 1976.
A frequent confusion concerns Sleeping Murder. That book was also written earlier and published posthumously in 1976, but it is a Miss Marple novel, not a Poirot novel. Curtain is therefore the final published novel featuring Hercule Poirot, even though Christie’s overall final novel was Postern of Fate in 1973.