I have read The Screwtape Letters more than once, and benefit from reading them every time.  I recently purchased the audio version (dramatized) from Focus on the Family.  It is simply one of the best audio theater type productions I have ever heard.  "Screwtape," the senior devil in this great book, is played by the gentleman who played Gollum in The Lord of the Rings trilogy.  He is astounding, and does an amazing job as Screwtape.  If you spend time in the car, or if you have children old enough not to be scared our of their wits (it is intense at times), this would be a great purchase.  On a more serious note, this work by Lewis is simply a wonderful work on the nature of the Christian life and the nature of temptation.  Ultimately, it is a excellent work on the spiritual life, and how to live a faithful life.  If you have never read it, put it on your list!