Which C.S. Lewis book was published posthumously in 1964 as a series of letters to Malcolm?

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

The book consists of fictional letters in which Lewis discusses prayer, worship, liturgy, and Christian practice. Its full title is Letters to Malcolm: Chiefly on Prayer. Lewis died in 1963, and the book appeared after his death in 1964.

The addressee, Malcolm, is generally understood as a literary correspondent rather than a straightforwardly documented real-life friend whose private letters Lewis reproduced. The informal letter format lets Lewis explore difficult religious questions conversationally, including how people pray, why public worship matters, and how language shapes belief.

This work is often confused with A Grief Observed, another posthumously published book associated with Lewis’s personal life. A Grief Observed focuses on grief after the death of his wife, Joy Davidman. Letters to Malcolm instead centers on prayer and was written in a different literary form.