Which C.S. Lewis book is a series of letters from a senior demon to his trainee nephew?

The Screwtape Letters is C.S. Lewis's satirical 1942 book presented as correspondence from the senior demon Screwtape to his inexperienced nephew Wormwood. The letters advise Wormwood on tempting a human subject.

Lewis uses the demonic viewpoint to expose self-deception, pride, distraction, and other habits he believed could distort moral and spiritual life. The unusual format lets readers infer the author's ideas from advice that is deliberately backwards.

The book first appeared during the Second World War and grew from letters Lewis wrote for publication in the Anglican newspaper The Guardian. It is not a conventional narrative novel, despite its fictional characters.