Sophisticated pointers for cooks who've exhausted (or shunned) Mary Ellen and Heloise. Pierot, the source earlier of Suzanne's Garden Secrets, is a member of Les Dames d'Escoffier, and such--and it shows. Her tips are not all new: we've known for years that spaghetti won't stick if you add a little oil to the pot, while we wouldn't think of adding salt to boiling corn. But some are indeed unusual (freezing chestnuts to make them easy to peel), and many are designed--with gourmet cooks in mind--to yield more-nearly-perfect results. Thus: how to tell when fish is completely cooked; how to keep cake frosting moist; how to fry parsley and keep it green. And Pierot doesn't skip the mundane altogether: have you thought of saving on paper towels, when you drain fried foods, by placing a layer of paper toweling on top of several layers of newspaper? The arrangement is orderly (by kinds of food); the tips are seldom gimmicks.