Theory of Poker is considered one of the best (if not THE best) foundation books ever written. It will teach you such a base level of the purpose behind poker math and concepts. However, I found it tougher to read as well. It's thick, so for a begining player it may be tough to retain.
Harrington on hold'em is excellent (both his cash game and tournament series), as well as Gordon's books. Yet for understanding the game I still think Ed Miller's Small Stakes Hold'em is one of the best. It's a limit game book, but for understanding math, aggression, the board, and hold'em in general, it will teach you tons. His NL book is great as well.
You always have Daniel N's books for quick 'snippit' reads, and Gus Hanson's All Hands Reveiled is also very nice. But if you want true foundation books for the game and theory of poker in general, Sklansky, Miller, and Mason Malmuth (sp) is still the top of the order.
P.S. An attempt to understand any one of these books with a single read is next to impossible imo. Reread and get on the tables, best way to learn.