Theory of Poker, Harrington on Hold'em 1 & 2, Tournament Poker for Advanced Players - probably in that order.

Keep in mind that TOP references many different games and is not specific to hold'em. It's the concepts and theories that you need to grasp in this book, not the exact examples and how they relate to lowball or razz, as will be the case in many examples, but how they relate to poker in general.