Which C.S. Lewis book presents Christian ideas through a series of letters from a demon?

The Screwtape Letters presents Christian ideas through letters from a demon named Screwtape to his junior tempter, Wormwood.

C.S. Lewis published the book in 1942. Each letter offers advice about tempting a human subject, but the demonic viewpoint lets Lewis expose self-deception, pride, distraction, and distorted reasoning. The book’s irony comes from allowing a senior tempter to describe harmful tactics as if they were sound professional guidance.

The work was first published in serial form in the Anglican newspaper The Guardian before appearing as a book. Lewis later said the project was not easy to sustain because the demonic voice made him feel spiritually uncomfortable. It remains one of his most widely read works, but it is different in form from Mere Christianity, which grew from radio talks aimed directly at a general audience.