Which C.S. Lewis work features the fictional demon Screwtape as a senior tempter?

The Screwtape Letters features the fictional demon Screwtape as a senior tempter.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape to his inexperienced nephew, Wormwood. Screwtape offers advice on how to tempt a human being away from Christian faith and moral discipline. Lewis presents the advice ironically, so the demons’ language exposes their selfishness, pride, and distorted reasoning.

The work began as a series of pieces published in the Anglican newspaper The Guardian. Lewis later collected them into a book. Although the letters are fictional and satirical, they address serious themes such as distraction, resentment, vanity, habit, and spiritual temptation. The title character is not a human villain or historical figure but an invented demon.