What was the name of C.S. Lewis's Christian allegory about a senior demon's advice?

The Screwtape Letters is C.S. Lewis’s Christian allegory presented as advice from a senior demon.

The book consists of letters from Screwtape, an experienced tempter, to his inexperienced nephew Wormwood. Instead of describing temptation from a human narrator’s viewpoint, Lewis reverses the perspective and lets demonic advice expose habits such as distraction, vanity, resentment, and self-deception.

Lewis wrote the work during the Second World War. The letters first appeared in the Anglican newspaper The Guardian in 1941 and were collected as a book later that year. The unusual format made the book accessible while also allowing Lewis to satirize moral and spiritual weaknesses.

The Screwtape Letters is sometimes grouped with Lewis’s straightforward apologetic works, but it is fiction and satire. Its narrator is deliberately unreliable: Screwtape’s claims reveal the demonic viewpoint rather than Christian doctrine.