My System by Nimzowitsch is a must have in my opinion. It teaches you how to play positionally. When you don't see any tactical opportunities, you must play positionally. It's kind of like, "Ahh, I don't see any good move, so I'll see what will better my position."
For tactics, Polgar's 5334 Problems, Games, and Combinations is definitely worth the money.
Strategy or tactic? The guy I usually play with online seems to always know where he's moving and why. I just kind of spontaneously decide after he's moved. Suffice to say, he always wins. I'm sick of this. I love chess. I want to improve. Anybody have suggestions for how I can improve (read book, etc. -- i found the video over my head.) Please, I can't let any man think he's superior to me. :)