I've accumulated a lot of cookbooks over the years, and they are usually in one of two types. General cookbooks like Joy & New Basics, and specialized cookbooks. I also have a number of books geared specifically to professional cooking as I used to be a chef.
The specialized cookbooks are my favorites because while they don't expand the breadth of my knowledge much, the expand the depth a great deal.
My favorite specialized cookbooks are:
Professional Pastry Chef by Bo Frieberg
Splendid Soups by James Peterson
Sauces also by Peterson
Bread Baker's Apprentice by Peter Reinhart
Bernard Clayton's new Complete Book of Breads by Bernard Clayton
I also have one pretty cool cookbook which is a general cookbook like Joy or Basics, but Italian.
Silver Spoon by Phaidon press has a basic version of pretty much every Italian dish you could ever think of, and 2,000 other ones too.