The 33 Strategies of War. - Robert Greene
I am too nice a guy. I am also the leader of my company. The two don't mix. You have to become somewhat detached, and somewhat of an asshole in order to effectively lead. I found out the hard way that sometimes you have to say to your people "because I freekin said so" instead of trying to be nice to them and let them make the mistake you know they are going to make (silly me, I forgot this was a business, not a community college).
The hardest thing for me, by far, has been to fire someone. I did it two months too late, and everyone in my company suffered for it. My lesson? Be an asshole at the right time.
I love the book I mention because the lessons apply everywhere in business: You need to motivate, you need to earn the respect of your people, you need to be decisive, you need to see the bigger goal, and you need to delegate correctly and put together the right structure in your company's leadership.
I've learned a lot from this book. Guess what? Our productivity is about 2-3 times higher now (no joke, 200% to 300% more work gets done every day). We're running fine, everyone is pulling their weight, and everyone is happy to be making professional progress. My own lesson from this book was that I had to get some balls, crack the whip, and turn from a fellow programmer to a leader and mentor. Your lesson from the book might be different, but I bet that almost everyone has something to learn from this book.