I just finished reading "The Jesus Prayer" by Frederica Mathewes-Green
This book is by my favorite podcaster on Ancient Faith Radio. The Jesus Prayer is an ancient prayer developed by the Desert Fathers. Quite simply, it's "Lord Jesus Christ, Son of God, have mercy on me, a sinner." It's a simple, but powerful prayer. This book, written in an easily readable style, talks about how to pray the Jesus Prayer and how it can change you.
You can see all the books I've read so far (starting in 2011) here.