Discrete Mathematics - Ross and Wright. Once you get into it, this shit is amazing. Epiphanies for days.
The Language Instinct - Stephen Pinker - an utterly fascinating book on language and the brain, chock full of 'well, shit' moments. He's also written a bunch more equally fascinating books - The Blank Slate and How The Mind Works being among them.
Rationality: From AI to Zombies This is a collection of essays on cognitive biases, fallacies and how to kid yourself that you're avoiding them. It will make your brain hurt.
The Philospher's Toolkit - Julin Baggini - a kind of cliff-notes introduction to a wide range of concepts in philosophy