Which C.S. Lewis work was published posthumously in 1964 as a collection of letters?

Letters to Malcolm: Chiefly on Prayer was published posthumously in 1964 as a collection of letters.

The book presents an imagined correspondence between Lewis and a fictional friend named Malcolm. Its letters discuss prayer, worship, suffering, Christian doctrine, and the practical difficulties of religious life. Lewis wrote the work during the final years of his life.

Lewis died in 1963, so the book appeared after his death. It was edited for publication and became one of his later theological works, alongside books such as The Four Loves and A Grief Observed.

The title can be confused with a conventional biography or with a collection of Lewis’s private correspondence. Malcolm is a literary correspondent created for the book, allowing Lewis to explore questions through a conversational exchange rather than a continuous treatise.