Which C.S. Lewis memoir describes his conversion to Christianity?

The C.S. Lewis memoir describing his conversion to Christianity is Surprised by Joy.

Published in 1955, the book recounts Lewis’s intellectual and spiritual development from childhood through his return to Christianity. It describes experiences of intense longing, which he connected with the idea of joy.

Lewis had been an atheist for part of his adult life before becoming a theist and then a Christian. His friendships with Christian writers, including J.R.R. Tolkien and Hugo Dyson, influenced the conversations surrounding his conversion.

The book is a spiritual autobiography rather than a complete day-by-day account. Readers sometimes confuse it with A Grief Observed, Lewis’s later, much shorter work about mourning the death of his wife, Joy Davidman.