Robert Cialdini's "Influence: Science and Practice." is great. I heard him speak in person once and it was amazing material, and he presented it very well. Unfortunately, some find his book to be a bit dry and a bit of a tough slog, but again, the material is great.

I can't emphasize enough how much his work has helped me. Here is a link to a quiz based on his material, which is very telling as to how good you are with respect to influencing others: http://www.influenceatwork.com/CialdiniQuiz.html

I actually read his book and heard him speak before taking the quiz and so I did much better on it than I would have otherwise.