What was the title of C.S. Lewis’s 1942 Christian apologetic book featuring a senior demon’s letters?

C.S. Lewis’s 1942 book featuring a senior demon’s letters is The Screwtape Letters.

The book is presented as a series of letters from Screwtape, an experienced demon, to his junior nephew Wormwood. Screwtape advises Wormwood on how to tempt a human patient, allowing Lewis to discuss human weakness, pride, distraction, habit, and spiritual life from an intentionally reversed perspective.

The Screwtape Letters first appeared in the Anglican newspaper The Guardian in 1941, and Macmillan published the book in 1942. Lewis dedicated it to his friend J.R.R. Tolkien, although their friendship later became strained.

The work is not a conventional narrative novel. Its epistolary form gives each letter the feel of a compact moral argument, and the demonic narrator often misunderstands the larger spiritual reality he claims to manage.