Probably David Sklansky's "The Theory of Poker." People tend to mention that book a lot, and for good reason. No matter what game you play, no matter if it's fixed, pot, or no limit there is solid advice nobody who wants to play well can ignore. Granted, I'm an academic type, and I never really played too much before checking out my first poker book so I can't say it changed my game. But it definitely shaped it. I would also argue the advice he gives in it has stood up better than any other poker book I know.