Which C.S. Lewis book was published posthumously in 1964 as letters about prayer?

Letters to Malcolm: Chiefly on Prayer was published posthumously in 1964 as letters about prayer.

The book takes the form of a series of letters from Lewis to a fictional correspondent named Malcolm. Its central subject is prayer, but the letters also consider worship, private devotion, liturgy, suffering, and the relationship between ordinary life and religious practice.

Lewis died in November 1963, before the book appeared. Publication in 1964 therefore made it one of his posthumous works. The conversational format distinguishes it from a systematic theological treatise and gives the book an intimate, exploratory tone.

Readers sometimes confuse it with The Screwtape Letters because both titles use the word letters. The earlier book is a satirical fictional exchange between demons, whereas Letters to Malcolm presents reflective correspondence about prayer.