What was C.S. Lewis’s final completed book?

Letters to Malcolm: Chiefly on Prayer was C.S. Lewis’s final completed book.

The book was published posthumously in 1964, after Lewis died in November 1963. It takes the form of letters to a fictional correspondent named Malcolm and explores Christian prayer, worship, suffering, church practice, and the relationship between private and public faith.

Lewis’s death meant that readers encountered the book after his lifetime, and some bibliographies therefore describe it as his last published book rather than simply his final work. A Grief Observed appeared earlier, in 1961, and was written after Joy Davidman’s death. The distinction matters because “last book” can refer either to the last work completed or the last one published while an author was alive.