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

The C.S. Lewis book consisting of letters from the senior demon Screwtape to Wormwood is The Screwtape Letters.

Lewis presents the work as correspondence written by Screwtape, an experienced tempter, to his inexperienced nephew Wormwood. The letters discuss how to lead a human away from faith and virtue. The satirical method reverses ordinary moral instruction: readers learn about temptation by hearing a demon describe it as professional strategy.

The book first appeared in Britain in 1941 and was published in book form that year. Lewis wrote it during the Second World War, when he was also becoming known for radio talks on Christianity. Its fictional infernal viewpoint is not a literal theological manual; it is a literary device used to expose self-deception, distraction, pride, and habit.

A common mix-up is to identify Screwtape as the human protagonist. He is the senior demon and letter writer, while Wormwood is his junior correspondent.