Food Hedonist's Essential
I worked in the hotel and restaurant world for almost 30 years and cooked tons and tons of food at many types of venues--fast food to fine dining to institutional. I was very good, but I never got any formal training. Sauces solves that problem, despite its focus on the French school of cooking. To be fair, I've just started deeply investigating the book and have yet to use any of its recipes, although it has influenced some of my recent creations.
Summing up, if you want to know how the chef made that decadent sauce for your entree, this book will have an answer for you.