Anselm of Aosta was an Italian-born English theologian, monk, and abbot at the Benedictine abbey of Bec served as Archbishop of Canterbury under William II from 1093 to his death on 1109. He is held to be a Saint in the Roman Catholic, Anglican, and Lutheran churches, and was proclaimed a Doctor of the Church by a bull of Pope Clement XI in 1720.
Anselm helped to inaugurate the Scholastic movement in the medieval period, sometimes credited as the "father of scholasticism," and was known for what is today referred to as the "ontological argument" for the existence of God.
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.