Behold the Spirit: A Study in the Necessity of Mystical Religion is one of the best books on theology I have ever read. Alan Watts separates the "earn your way to heaven" theology of the legalistic religions from the simple gospel of grace and redemption found in the Bible.
The author has a gift of taking a complex subject and making it understandable to a lay person. If you are a non religious person but are curious to know what Christianity is all about then read this book. If you are religious, read this book for an in depth exposition of the theology you believe. You will learn something.