My #1 is going to have to be Tartine Bread by Chad Robertson. It was my introduction to real sourdough, and the basic Country Loaf recipe is a staple in my house.
Flour Water Salt Yeast, by Ken Forkish, is the one I go back to over and over again to understand the why and how of bread baking. "Why does it react this way?" or "How do I modify this recipe?", the explanations and techniques are something I find very helpful.