The Screwtape Letters by C. S. Lewis
I love C.S. Lewis's unique perspective in this book (writing from the perspective of a devil). There is a lot of food for thought for Christians (live in the present, it's the "little" sins that cause us to stumble, one of Satan's primary goals is to cause us to be self-focused, etc.). At times I got bogged down by the reading; it seemed to drag a bit--could be partially due to Lewis's academic-type sentence structure and vocabulary. The chapters, however, are very short, so it was easy to take a break when I needed one.
This book was part of my 2010 summer reading program.