In New York chef Boulud's world, the flavor of a dish is like a good story: It has a beginning, a middle, and an end. If aspiring cooks understand that narrative, they've taken a solid step toward success in a professional kitchen. And as the Frenchman makes clear in ''Letters,'' knives must be sharpened daily, onions precisely chopped, and heat mastered. (''When you understand heat, you 'see' food down to its very molecules.'') Even a simple omelette is a study in dedication, purity, and skill. You don't have to be a culinarian to love this book, which offers invaluable insight for dedicated diners and Food Network junkies, too.