Depends what games you're playing exactly, but here's what I've heard is good:Super System 1&2 (Doyle Brunson)David Sklansky's Theory of Poker and Hold Em Poker (Sklansky is consider THE expert on anything poker, especially in regards to theory)Internet Texas Hold Em (Matthew Hilger)The Complete Book of Hold Em Poker (Gary Carson)Hold Em Excellence: From Beginner to Winner (Lou Krieger)Winning Low Limit Hold Em (Lee Jones; this one has been recommended to me many times since I also play low limits)Hope that helps.Also, just looking up strategy in general will give you a lot of great tips. There are lots of great articles out there, many by professional players, completely free.