Moral Politics, by George Lakoff: this book offered a new perspective on not just the discussion and language of politics, but sent me down the rabbit hole of cognitive linguistics and "embedded mind" in books he authored or co-authored, such as "Metaphors We Live By", "Philosophy in the Flesh", and "Where Mathematics Comes From".

Guns, Germs and Steel by Jared Diamond: Gave me a deeper insight into the geographic underpinnings of civilization and history, and how available resources and climate allowed some early settlements to grow into mighty empires, while others struggled to subsist. It also improved my Civilization 3 game playing. ;)

The Language of Mathematics by Keith Devlin: Sparked a renewed interest in mathematics through historical narratives of the development of our number systems, mathematicians and numerous mathematical topics.

The Code Book by Simon Singh: Kicked off my historical and professional interest in cryptography and keeping secrets.

Flow by Mihaly Csikzentmihalyi: Showed me how to make the best use of every minute, especially waiting at bus/train stops, and helped me recognize what I really like doing.