Selfish Gene allowed me to straighten out some of my preconceptions – and there aren’t too many books I’ve come across that can make that happen.
Dawkins is exceptionally rational, and calmly convincing in his arguments (a great variance in tone from his later work The God Delusion). I particularly enjoyed his exploration of Game theory – with “evolutionarily stable strategies” analogous to “Nash Equilibriums”. It will change the way I look at interactions between organisms (including people) for months to come.
The book is not hard to follow, regardless of your background. I would recommend this to anybody with even a passing interest in biology.