For me, I found How to Win Friends and Influence People by Dale Carnegie and Getting To Yes by Bruce Patton to be invaluable. They taught me more than just how to communicate, but how to work with others who have different goals or perspectives than I have. I really started to view some managers differently after reading the book and was able to get more of what I wanted (the correct technical solution) while still giving them what they needed (the feeling of power and authority) at the same time. Although it doesn't sound like it fits in this thread, understanding someone's perspective and motivation enables communication that is not just diplomatic, but actually leads to mutual benefit. Of course, your mileage may vary (hopefully you don’t have a manager whose need is to see others suffer needlessly….)