Which C.S. Lewis book uses the fictional demon Screwtape to give advice to a junior tempter?

The C.S. Lewis book in which the demon Screwtape advises a junior tempter 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 his nephew Wormwood. The advice concerns temptation, distraction, pride, fear, and the manipulation of human habits.

Lewis uses the imagined viewpoint of Hell to create irony: readers are expected to recognize the distortions in Screwtape’s reasoning. The book is therefore not a handbook for evil but a satirical Christian exploration of moral and spiritual weakness.

The work began as pieces published in the Anglican newspaper The Guardian during 1941. Lewis later collected and revised them for book publication. Its unusual epistolary form distinguishes it from Lewis’s more direct apologetic works such as Mere Christianity and The Problem of Pain.