Don’t Shoot the Dog by Karen Pryor. This book and the next book, Culture Clash, are considered “classics” when it comes to discussing the learning theory behind current dog training methods. They are both very good if you really want to dig into the details of how/why certain methods work. If you really just want a well-behaved dog and don't care so much about the how and why, then these are not for you. These are for the people who really want to know all the details!
Don’t Shoot the Dog by Karen Pryor