I'm still such an amateur, but a whole lot better than I used to be.
It seems like my cooking will plateau, and then...I discover a new cookbook and learn something that takes everything up a notch.
Working with Patricia Wells's Trattoria and Bistro Cooking did that for me. Left me with a much better intuitive feeling for how certain cooking methods work (properly salting pasta, using bouquet garnis, etc.). Same with Yan-kit So's Classic Chinese Cookbook -- so precise on cooking methods and what you're trying to achieve. America's Best Recipes has done that for me with some baking techniques.
So I'm curious...what cookbooks have transformed your cooking and made you a more competent, confident cook?
I'm particularly interested in bread baking, if anyone has suggestions.