Oh how I love C. S. Lewis! My all time favorite book is The Screwtape Letters. You might think that odd. I consider it pure brilliance. Once you fully understand how Satan and his demons work and connive, and you fully understand the spiritual warfare in which we, as Christians, find ourselves, you then take seriously the things of the Spirit (Holy Spirit, that is).
Focus on the Family Radio Theatre is coming out with The Screwtape Letters. Go here to see a fantastic video clip.