Which book by C.S. Lewis was adapted from his 1941-1944 BBC talks on Christian belief?

Mere Christianity was adapted from C.S. Lewis’s BBC talks on Christian belief broadcast between 1941 and 1944. The talks were later revised, expanded, and collected into a book.

The work is arranged in four books: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” “Christian Behaviour,” and “Beyond Personality.” Lewis presents arguments about morality, Christian doctrine, and the nature of Christian life in a style designed for general listeners.

The title does not refer to a separate denomination called Mere Christianity. Lewis used the phrase for what he saw as the common ground shared by historic Christian traditions. The book became one of his most widely read works, alongside The Chronicles of Narnia and The Screwtape Letters.