I would agree with DrPhysic on Zen and the Art of Poker although I believe most 2+2ers give this book average marks. It was helpful for me when I was just getting started out. It may not be as helpful for those playing for a long time as a lot of it may be common sense by then. Anyways, give it a try, the book is on the cheap side.

I haven't read it, but a lot of people also recommend Psychology of Poker by Alan Schoonmaker (sp?). It's a 2+2 book, so you can get more information on this site.