What is the name of C.S. Lewis’s Christian allegory about a demon advising his nephew?

C.S. Lewis’s Christian allegory about a demon advising his nephew is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters presents a series of fictional letters from Screwtape, an experienced demon, to Wormwood, a novice tempter. Screwtape offers advice about distracting a human subject from faith and moral action. Lewis uses the reversed perspective to expose habits such as self-deception, resentment, vanity, and spiritual laziness.

The book first appeared as installments in the Anglican newspaper The Guardian during 1941. That publication was unrelated to the modern British newspaper of the same name. Lewis later dedicated the book to J.R.R. Tolkien, though the two friends disagreed about aspects of its style. The work is often grouped with Lewis’s apologetic writing, but its epistolary form and dark comic voice make it distinctive.