If you'd like to play limit hold'em, I'd recommend a couple books. The best book I ever read in terms of money it made me is Small Stakes Hold'em
. This book is very good at showing you how to beat loose limit games. This would be most $2/$4 games-$4/$8 or $5/$10 live. Much lower stakes online, I'm honestly not even sure how low.
If you're playing online, in the games that are more TAG, then you might want to check out Hold'em Poker for Advanced Players (http://www.amazon.com/HoldEm-Advanced-Players-Advance-Player/dp/1880685221/ref=sr_1_2?ie=UTF8&s=books&qid=1262583385&sr=8-2), which is more geared toward playing styles that will succeed in this tougher game environment.
Either of these books can teach you a lot about positional play, how action in front of you should be considered, and a multitude of postflop intricacies. If you were only going to read one, I would definitely steer you toward the Small Stakes book, because there is information on playing tighter games as well, but overall it's just a very very solid book on the game. I haven't read HPfAP nearly as thoroughly or recently, but I do know the information in it is still quite good.
I'm open to questions too. I've played quite a bit of limit, so I can likely answer just about anything you might want to know.