Which C.S. Lewis book explains Christian belief through a fictional conversation between a demon and his nephew?

The Screwtape Letters explains Christian belief through fictional letters written by the senior demon Screwtape to his nephew Wormwood. C.S. Lewis published the book in 1942.

The letters reverse the usual moral viewpoint: Screwtape advises Wormwood, a junior tempter, on how to lead a human away from faith and virtue. Lewis uses this ironic perspective to discuss temptation, distraction, pride, prayer, love, and spiritual discipline.

The book began as a series of letters published in the Anglican newspaper The Guardian. Lewis later collected them as a book, adding a preface that explains the difficulty of writing from a demonic viewpoint. The unusual format made the work accessible while also allowing sharp criticism of human habits.

The Screwtape Letters is distinct from Mere Christianity, which developed from wartime radio broadcasts, and from The Great Divorce, which uses a dreamlike journey to explore heaven, hell, and moral choice.