Which C.S. Lewis book presents the fictional demon Screwtape as a senior tempter?

The C.S. Lewis book that presents Screwtape as a senior tempter is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape, an experienced demon, to his junior nephew Wormwood. The advice concerns how to influence a human subject, whom Screwtape calls “the Patient,” away from Christian belief and moral attention.

Lewis uses the inverted viewpoint as a satirical device. The demons’ professional language and self-interest expose habits such as distraction, vanity, resentment, and complacency. The work is not a conventional story told by a human narrator; its epistolary format drives the satire.

The book grew from a series of pieces first published in the Anglican newspaper The Guardian. Lewis later explained that writing from the demonic perspective was difficult and unpleasant, which is one reason he did not continue the series as a conventional sequel.