Which C.S. Lewis book was written as a fictional diary by a senior demon?

The Screwtape Letters was written as correspondence from a senior demon.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. Screwtape advises Wormwood on how to tempt a human patient and undermine the patient’s spiritual life.

Lewis uses the inverted perspective as satire: readers must infer the author’s moral and theological meaning from advice that is deliberately malicious. The letters address distractions, pride, vanity, fear, politics, relationships, and spiritual discipline in a sharply comic style.

Although often described as a fictional diary, the work is more precisely an epistolary collection of letters. That distinction matters because the book’s structure depends on the relationship between Screwtape’s instructions and Wormwood’s unseen responses. It remains one of Lewis’s most recognizable works outside Narnia.