Which C.S. Lewis book presents conversations about Christianity between two fictional correspondents?

The C.S. Lewis book that presents conversations about Christianity between fictional correspondents is The Screwtape Letters.

Published in 1942, the work consists of letters from Screwtape, an experienced demon, to his nephew Wormwood, who is trying to tempt a human being. The device lets Lewis discuss temptation, pride, distraction, prayer, love, and moral choice from an intentionally inverted viewpoint.

The book is not an exchange of ordinary letters between Christian teachers. Its humor and irony come from hearing spiritual advice delivered by demons, while readers infer the Christian principles Lewis is defending. The imagined “patient” is never named, which keeps attention on the techniques of temptation rather than on a single biography.

Lewis dedicated the book to J.R.R. Tolkien, though the dedication was later removed from some editions. The title character's name is often misspelled as “Screwtape”; the spelling is important because it is the book's central proper name.