The SFist test kitchen staff has a tiny list of essential food books. At the top you'll find Harold McGee's . Our copy's tattered cover and stained pages broadcast our love for this accessible bible of kitchen science. When we use the bright red tome to answer our culinary questions, we say we're "Reading From the Book of Harold." Alton Brown uses this book as a reference.
Why should you click the above link and buy your own copy? Because the book gives you the tools to understand cooking and flourish as a chef.
Consider vinaigrettes. You could parrot a food magazine's recipe, or you could learn about them from On Food and Cooking. You'll learn how to make a quick version (combine 3 parts oil, 1 part vinegar, and flavorings; shake vigorously at the last minute and dress the greens). Read a little further and you'll learn about emulsions, and why this type of vinaigrette covers greens more effectively than the whisk-oil-into-vinegar type. You'll get some practical advice to make sure the dish doesn't go awry (dry your greens well, since water and oil are "antagonists"). Read still further and you'll realize that the basic vinaigrette method applies to any number of fats and liquids, and you'll learn how other modern chefs have created their own variants, no doubt using the principles they learned from the first edition.
Photo courtesy Harold McGeeOn Food and Cooking