Which C.S. Lewis work is a collection of letters to a fictional young demon?

The C.S. Lewis work that is a collection of letters to a fictional young demon is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters presents advice from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. The letters reverse the expected viewpoint: instead of describing temptation from a human perspective, Lewis imagines how demons might discuss the corruption of a human subject.

Lewis uses satire to explore pride, distraction, resentment, self-deception, and the gradual formation of moral habits. The work is fictional, but its arguments reflect Lewis’s Christian understanding of temptation and spiritual life. Screwtape’s confident tone is part of the joke, since his advice exposes the absurdity and destructiveness of demonic reasoning.

The title is sometimes shortened informally to “Screwtape,” but the canonical book title is The Screwtape Letters. Lewis later wrote a preface explaining aspects of the book’s unusual form and warning readers about the dangers of treating evil too casually.