From the Extraordinary Mind of C.S. Lewis, IV (or, Screwtape)

The Screwtape letters was the book he least enjoyed writing, although it was the easiest for him. For me, it was an easy read, but a disturbing one at that. The book deals with the reality sometimes I pretend doesn't exist--there are demons right now, plotting my downfall very carefully.

Here is the sequel Lewis never wanted to write, but wrote anyway. I highly recommend it.

http://www.saturdayeveningpost.com/wp-content/uploads/satevepost/screwtape-proposes-a-toast-SEP.pdf