Regardless of what you play, grab a copy of Theory of Poker by David Sklansky. Then if you're playing No Limit Hold 'Em, a good newbie book is Getting Started in Hold'em by Ed Miller. Also grab No Limit Theory and Practice (Written by Miller and Sklansky) and also get Professional No Limit Hold'em. Those are pretty much the only no limit books around.

For LIMIT hold 'em, grab Small Stakes Hold'Em (Miller, Malmuth, Sklanksy), Hold'em Poker For Advanced Players (Malmuth, I think), and Weighing the Odds in Hold'em Poker (King Yao)

Those are all pretty much the "required" readings.