You’re fluent in fruit-acid peels, you’ve got a master’s in moisturizers, and your bathroom looks like the Crème de la Mer warehouse – but you still don’t know if your skin is oily or dry? Now Dr. Barney Kenet, a Manhattan dermatologist who has treated Demi Moore’s overexposed epidermis as well as that of chain-smoking models and workaholic Wall Streeters, offers a new instruction manual, How to Wash Your Face (Simon & Schuster), to help you figure it out. Dr. Kenet gives directions like a drill sergeant: Cut shower time to five minutes; you’re washing too frequently; and no, for the last time, nothing cures cellulite. Oh, and one more thing. “It’s not just about buying products,” says Kenet, confirming drugstore junkies’ worst fears. “Your skin is a representation of what is going on inside you.”