The Screwtape Letters, by C. S. Lewis
As long as I was reading about suffering and the meaning of life and all with Viktor Frankl, I picked up the Screwtape Letters again as well. I'm pretty sure we should all be reading this book every couple of years, regardless of religion, because Lewis is just so good at understanding human nature. I run into myself every couple of pages.
Since it's one of the most famous books around, it probably doesn't need me to explain it, but....Lewis writes a comedic and sharply insightful set of letters analyzing human nature--from the point of view of a devil in the business of tempting. One very nice element is the image of Hell; as a member of the century that saw the rise of organized, bureaucratic genocide, Lewis paints Hell as a massive bureaucracy, with the office workers all quietly scheming to do each other down.
Every letter from Screwtape to the lesser demon Wormwood discusses some point about humanity--and very frequently I end up reminded of something that I ought to pay attention to. As I said, I'm always running into myself in these pages, and usually in a way that shows me that I can improve a little bit somewhere.
If you've never read The Screwtape Letters, do so. It's not everyone's cup of tea by any means, but it's good fun, very short, and disconcertingly sharp. There have also been some excellent audio versions made; in particular, you might like to listen to John Cleese narrate it. It's on youtube, and here is my favorite chapter, the Eighth Letter.
ETA: I just recieved this photo of the back cover of an early edition, which features a sketch of Screwtape as Lewis imagined him.