Which C.S. Lewis book presents Christianity through letters between a senior and junior demon?

The C.S. Lewis book presented as letters between a senior and junior demon is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters consists of correspondence from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. The letters discuss temptation, distraction, pride, doubt, and human weakness from the imagined perspective of Hell. Lewis used this reversed viewpoint to make readers examine ordinary moral choices rather than to provide a conventional narrative.

The work first appeared serially in the Anglican newspaper The Guardian, which should not be confused with the modern British newspaper of the same name. Lewis later adapted the material for stage and radio. The book remains one of his best-known Christian apologetic works, but its epistolary format and satirical tone distinguish it from Mere Christianity, which grew from wartime radio broadcasts.