Once upon a time there was a young woman who longed for a fairy godmother. But not just any fairy godmother, rather one with specific talents and preferably a medical degree. For what this heroine (of her own mirror) really wanted was an all-knowing mentor who would guide her through the thickets of department store frist floors and out of the mazes of dimestore and drugstore cosmetic counters, placing the proper potions in her and hand as they went. "Buy this, don't buy that. Follow this regiment, not that one "Thus, ideally, would Wisdom counsel Vanity.

Now it looks as if our young woman can live beautifully ever after. Or, if not exactly that - well then, at least released from the high-priced spells cast by Estee and Elizabeth and Helena and Charles. A real, live doctor has ridden up on a white charge, brandishing a book: Jonathan Zizmor, M.D., dermatologist fairy godfather.

Dr. Zizmor, it must be stressed, has not written a book intended strictly for either ugly ducklings or swans; he does not promise transformations. He does present a straighforward accounting of hair and skin, tooth and rail, bath and shower products for all consumers interested in obtaining the best value for the least money. The use of brand-names throughout lends to his advice a satisfying, slightly illicit flavor (like learning the facts of life from a friend who doesn't mess around with euphemisms).

What he's against: paying through the nose for the expense of the manufacturer's advertising and packaging (and the snobbery held against many inexpensive labels; lime scents; mouthwash( twin blades, artificial nails, daily blow-drying feminine hygiene preparations, and shampoos with useless fruit and vegetable additives. He favors colognes and toilet waters over perfumes, sunscreens containing "PABA"; tooth powders, and most plain old soaps, with hazel, and Vaseline.

Zizmor's iconoclasm is as refreshing as any astringent and just as bracing as an aftershave. Given the way he cuts through the confusions of the marketplace, he deserves to be a household word. (Harper & Row, $12.95)