The Great Divorce is my favorite C.S. Lewis book; in fact, one of my favorite all-time books:

http://www.amazon.com/Great-Divorce-.../dp/0060652950

Yes, I've read Screwtape and most of his other books too, but many years ago (1970s mostly). I liked how they did the Lion, Witch & Wardrobe movie recently; hope they do the other books too.