Two come to mind right away. The first is Baking Illustrated from the folks at Cooks Illustrated. It is a veritable tome of information! A bit cumbersome, in my opinion, for quick reference, but I couldn't do without it.
The other one that I love is Gale Gand's Butter, Sugar, Flour, Eggs. Great recipes, techniques, photos, tips, etc. A nice feature is that a beverage is recommended for each dessert. Great idea since it's something that is easily overlooked!
is a great place to by cookbooks. I've found that the have very good prices.