The Flavor Bible - I know I pick this book up once at least once a day… its packed full of information, which flavors from which ingredients go with what. A great source of inspiration, especially when you have, say, only endive, lemons, and fava beans in the fridge.
The Professional Chef - My textbook for school. Super heavy, but useful. Only downside is that most of the recipes are listed as serving 20+, as its for the professional classroom or restaurant, so you will have to do some math to scale down.
Gastronomy of Italy - One of the oldest cookbooks I own. Anna, the author really has a deep love and understanding of real italian cooking. All of the recipes are easy to follow, authentic and come with either a cultural or historical explanation, which is always interesting