I don't feel that I'm qualified to name "must-read" fiction books, but here's a non-fiction book that everyone should read:
Emotional Intelligence by Daniel Goleman
It starts out dry, with some neurological explanations, but it eventually delves into how emotions come into play in people's decision-making, and how people can and should interact with each other.
Also, I read it immediately after Malcolm Gladwell's "Blink," and it covers many of the same topics, but it has more depth and breadth.