Which C.S. Lewis book was published posthumously in 1964 as a study of prayer?

Letters to Malcolm: Chiefly on Prayer was published posthumously in 1964 as a study of prayer by C.S. Lewis. The book is presented as a series of letters addressed to a fictional correspondent named Malcolm.

Lewis discusses private prayer, public worship, the Eucharist, suffering, and the difficulty of maintaining attention. The conversational letter form lets him explore theological ideas through personal reflection rather than a systematic academic treatise.

The book appeared after Lewis died in 1963. It is sometimes confused with A Grief Observed, another late work that appeared under a pseudonym and focuses on bereavement. Letters to Malcolm is instead centered on Christian prayer and was edited for publication after Lewis’s death.