The standard books are
Theory of Poker by Sklansky, very very good but examples are from games besides holdem
No Limit Holdem Theory and Practice by Sklansky and Miller, again very good.

There's also alot of good ebooks available like
Easy Game by Balugawhale
Small Stakes No Limit Holdem by Ed Miller
The Poker Blueprint by Tri Nguyen

These are all good books from what I've read.