It's the how to of cooking.
Most recipes are variations on a theme.
The reference book is where you get the basic foundations to build on, and learn cooking skills that will last a lifetime. This is where to find out the difference between cumin and coriander or how to cook an omelet. The new American edition is a short version of the original. This used to be the authoritative tome on cooking when the French methods still dominated the American gourmet scene, and gives fascinating information on the history and curiosities of cooking. It might appear a bit outdated since the advent of fusion cooking, and doesn't feature ethnic recipes.