On Food and Cooking — Harold McGee

Not a cookbook in the traditional sense — it's a 884-page encyclopedia of food science. McGee explains the chemistry and physics behind every cooking process: why bread rises, how caramelization differs from the Maillard reaction, what happens to an egg at every degree from 130°F to 212°F. Chefs from Heston Blumenthal to Thomas Keller cite it as the book that changed how they think about cooking. If you want to understand food rather than just prepare it, this is the definitive reference.