Which C.S. Lewis book is presented as letters from a senior demon to a junior tempter?

The Screwtape Letters is the C.S. Lewis book presented as letters from a senior demon to a junior tempter.

Published in book form in 1942, the work consists of advice written by Screwtape to his inexperienced nephew Wormwood. The demons discuss how to influence a human subject, whom they call “the Patient,” using distraction, vanity, resentment, and other ordinary temptations.

Lewis uses the fictional demonic correspondence as a satirical way to examine human behavior and Christian ideas about temptation. The letters are not written as a conventional narrative with a continuous plot; their structure comes from the correspondence itself.

The book first appeared in installments in the Anglican newspaper The Guardian. Lewis later collected those pieces into the volume now widely known as The Screwtape Letters.