Silmans complete endgame manual, not because it's the funniest book I've read, not because it's the book I've read most. And not because it's the book that has helped me improve the most (although it has helped me grab quite a few points) but mainly because it's so practical. There is just so much information in it, it's cleverly organised and it's very instructive.
What is your favorite chessbook and why?
Mine right now is "The Art of Learning" by: Josh Waitzkin. "The Art of Learning" is my favorite chess book because I am involved in both chess and martial arts as is Josh Waitzkin and it provides analyze into learning pretty much anything but mainly chess and Martial Arts.