The Screwtape Letters by C.S. Lewis
I love this book - it really makes you think. For those who have not read it, the book is written as a compilation of letters from a "tempter," Screwtape, to his nephew, a "junior tempter" named Wormwoood. In the letters, Screwtape gives Wormwood adivce and counsel on how to best tempt his "subject" - a young man who converts to Christianity, and then falls in love with a Christian woman. Through the letters, you are constantly reminded and made to think about how the adversary tempts us. What is truly excellent about the book, though, is that the cunning plans are not centered around obvious sins, that so often are what we think about when we think about temptation and sin. Instead, the tempters focus on much more subtle forms of sins - vanity, pride, distraction, insincerity, forgetting God, and how these can achieve the same effect as more obvious sins ... to lead us away from God. Ultimately, the tempters in this story do not care what sins are committed by their subjects - so long as they accomplish their goal of separating people from God, and leading them to the adversary. In fact, they seem to prefer the more subtle means of leading people astray, as they sense that this is a more hidden and thus secure way to accomplish their ultimate design.
You cannot read this book and not think of how extremely pertinent it is to your life. C. S. Lewis has thought deeply about the things we do each that lead us away from God, and he articulates them very well. As you read the book, you are in a constant introspection of your own life, and the things that are put before you daily that lead you away from what we all desire - a close, personal, consistent, and deep relationship with God, that leads to happiness now and the hereafter. I love this book!