Which C.S. Lewis book presents Christian ideas as a fictional correspondence between demons?

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

The letters describe human temptation from the demons’ perspective. Their satirical advice is intended to expose spiritual and moral weaknesses, including distraction, vanity, resentment, and self-deception. The unusual viewpoint lets Lewis discuss Christian ethics without presenting the work as a conventional sermon.

Screwtape is a fictional character, not a figure from the Bible. Lewis invented the correspondence as a literary device, and the book’s humor depends on the demons treating harmful behavior as professional success. The work became one of Lewis’s most popular theological books.