Ratio: The Simple Codes Behind the Craft of Everyday Cooking, by Michael Ruhlman.
This book is a guide to creating recipes that work. It's not a cookbook in the sense that it is full of recipes, it's a tool you can use to create your own.
When you know a culinary ratio, it’s not like knowing a single recipe, it’s instantly knowing a thousand.
Ratios are the starting point from which a thousand variations begin.
Ratios are the simple proportions of one ingredient to another. Biscuit dough is 3:1:2—or 3 parts flour, 1 part fat, and 2 parts liquid. This ratio is the beginning of many variations, and because the biscuit takes sweet and savory flavors with equal grace, you can top it with whipped cream and strawberries or sausage gravy. Vinaigrette is 3:1, or 3 parts oil to 1 part vinegar, and is one of the most useful sauces imaginable, giving everything from grilled meats and fish to steamed vegetables or lettuces intense flavor.
Cooking with ratios will unchain you from recipes and set you free. With thirty-three ratios and suggestions for enticing variations, Ratio is the truth of cooking: basic preparations that teach us how the fundamental ingredients of the kitchen—water, flour, butter and oils, milk and cream, and eggs—work. Change the ratio and bread dough becomes pasta dough, cakes become muffins become popovers become crepes.
It's the only "cookbook" I consult regularly.