Which C. S. Lewis book uses letters from a senior demon to advise a junior tempter?

The C. S. Lewis book using letters from a senior demon to a junior tempter is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters is a satirical Christian work presented as correspondence from Screwtape to his inexperienced nephew, Wormwood. Screwtape offers advice on tempting a human patient and undermining the person’s moral and spiritual life.

The unusual viewpoint lets Lewis discuss ordinary weaknesses such as distraction, vanity, resentment, and self-deception. Because the advice comes from a demon, the book often works through irony: readers are expected to recognize that the demons’ goals are destructive and that their descriptions expose human habits.

Lewis dedicated the book to J. R. R. Tolkien, although their friendship later became strained over the dedication. The work began as a series of pieces published in the Anglican newspaper The Guardian before being collected. It remains one of Lewis’s most recognizable experiments in literary form.