I love Radical Brewing for crazy ideas for recipes and weird ingredients. This is probably my favorite book.
How to Brew for technical info.
The New Complete Joy of Homebrewing for beginners because I think it is a better starter book.
Various style books for the appropriate styles.
Lots of recipe books that I look at for ideas, but Brewing Classic Styles is my favorite for great recipes to start from.
I haven't delved in to any of the new books yet, they're in the pile though