Thanks for all the suggestions so far. I'll throw in several more favorites that have changed the way I think over the years, in no particular order:

* The Righteous Mind - Jonathan Haidt

* 7 Habits of Highly Effective People - Stephen Covey

* The Emperor of all Maladies - Siddhartha Mukherjee

* The Alchemist - Paulo Coelho

* Getting Things Done - David Allen

* The Worthing Saga - Orson Scott Card

* The 4-Hour Work Week - Timothy Ferriss

* The 5 Love Languages - Gary Chapman

* The Total Money Makeover - Dave Ramsey