Phil Gordon's Little Green Book
I like this one currently. It focuses on playing NL holdem with more aggression in your play.
Betting bigger, check raising, etc. to find out information, focuses on playing position... He has suggested bet amounts that he would do in certain situations too.
If you are conservative by nature and aren't quite sure how to be more aggressive successfully, this is a good book to read. It'll teach you how to vary your play a bit so you aren't so predictable to your fellow poker players.
Cheers,
Vegas Cat