Which Agatha Christie novel was published posthumously in 1976?

The Agatha Christie novel published posthumously in 1976 was Sleeping Murder.

Sleeping Murder: Miss Marple’s Last Case was published in the United Kingdom by the Collins Crime Club in October 1976, after Christie’s death in January of that year. It was her final published novel and features Miss Marple investigating a long-hidden murder.

Christie had written the book years earlier, probably during the Second World War, and placed it in secure storage with another unpublished manuscript. She arranged for it to be released after her death so that a final Miss Marple mystery would remain available to readers.

The common mix-up is with Curtain, which was also published posthumously, in 1975, but it is Poirot’s final case. Postern of Fate appeared in 1973 during Christie’s lifetime, while The Harlequin Tea Set is a posthumous short-story collection rather than the novel described here.