Fisher, R., Ury, W. L & Patton, B. (2011). Penguin.
Everyone who ever has to deal with other people would benefit from reading this. (In other words, everyone.) It explains how to separate people issues from the problem, focus on interests rather than positions, and work together to create options that satisfy all parties. Though my work as a business analyst doesn't typically require this sort of opposed negotiation, I found a lot in here that will be useful in discussions with stakeholders over their needs and priorities. I'll also want to read it again the next time I'm considering the purchase of a home or car.