Which C.S. Lewis book was published posthumously in 1964 after his death?

Letters to Malcolm: Chiefly on Prayer was published posthumously in 1964 after C.S. Lewis’s death.

The book is presented as a series of letters about prayer, worship, Christian belief, and spiritual practice. Its imagined correspondent is Malcolm, and the conversational format lets Lewis explore difficult religious questions without presenting the work as a formal treatise.

Lewis died on 22 November 1963. Letters to Malcolm appeared the following year, making it one of the major works released after his death. It was edited for publication from material Lewis had prepared.

The title can be confused with collections of Lewis’s actual correspondence. It is not a biography of a person named Malcolm, nor is it a novel. Instead, it belongs to Lewis’s Christian nonfiction and continues his practice of explaining theology through personal, accessible prose.