Ratio: The Simple Codes behind the Craft of Everyday Cooking by Michael Ruhlman
This is the coolest book for cooks. I've needed this so many times in my life. I simply find it oppressive to race out to the store every time I am missing something in a recipe, so I have often just wanted to know what the ratios are--what is absolutely necessary for something to be defined as "cake" as opposed to "bread" for instance--so that I can create. And here it is. I'm so glad.