Proslogion:
This short book is a defense of God based on the ontological argument. From what I understand, St. Anselm basically came up with this argument. The premise is that we can reason who God is and what He is by reasoning Him out - by sola ratione. It is interesting to think about, but has several glaring flaws, not the least of which is that it completely ignores the doctrine of total depravity - that man cannot understand, let alone choose, good without grace from above. Of course, since St. Anselm was a Catholic, he likely didn't believe in this doctrine anyway.
I thought this book was intriguing, and it offered some good discussion in our study group. I would not recommend it to a casual reader though; there are many better books to read theologically-speaking. But as an educational exercise, go for it.