Sidney Petrie is as oily and seductive as Svengali, only not so ambitiously evil. His ""magic-relaxer monologue"" is directed toward turning down the overeater's appestat and turning him off ""the no-no foods"" -- white flour and sugar. Relax, he says; also ""Absolve, Forgive, Praise, Remind."" You'll spend a lot of time mumbling positively to yourself and playing with a pendulum Petrie says you have attached directly to your subconscious, where lurk the character assassins of your self-esteem. ""Your want adjusts,"" he promises. The publishers also include a 33-1/3 RPM recording of Sidney's routine, free with every book. Now that's marketing.