For me it was The Art of War by Sun Tzu. It made me see how human conflict is a part of our nature that I needed to know better, and perhaps embrace a little at times. The book of course is about how to win in ancient warfare, but it explained this in universal terms that get to the heart of how humans engage in any sort of conflict. I've then gone on to read other great books, but this began a major evolution in my thought process about many things.