Which C.S. Lewis book presents Christianity through letters from a senior tempter?

The C.S. Lewis book featuring letters from a senior tempter is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters is a Christian apologetic work written as a series of fictional letters. Screwtape, an experienced demon, advises his inexperienced nephew Wormwood about tempting a human being known as “the Patient.”

The unusual viewpoint lets Lewis discuss temptation, pride, distraction, fear, and spiritual habits through satire. The book is not presented as a literal account of an underworld bureaucracy; its demonic correspondence is a literary device for examining human behavior.

A common mix-up is to identify The Great Divorce, another Lewis work, as the letter book. The Great Divorce is instead an allegorical tale about a bus journey from a version of Hell to the outskirts of Heaven.