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

Letters to Malcolm: Chiefly on Prayer was published posthumously in 1964.

The book takes the form of a series of letters about prayer, worship, and Christian belief. Lewis died in November 1963, before the work appeared, so it was published after his death in 1964. The imagined correspondent, Malcolm, gives the discussion an informal and personal structure rather than the style of a conventional theological treatise.

Lewis addresses practical questions about private prayer, public worship, distraction, and the relationship between human language and divine reality. The book is among his later religious writings and reflects concerns that also appear in his earlier apologetic works, while remaining more conversational in form.

It is sometimes confused with The Discarded Image, another book published after Lewis’s death, in 1964. That work concerns medieval and Renaissance literature and cosmology. Letters to Malcolm is specifically centered on prayer and is identifiable by its epistolary format.