COOKERY Irrepressible fitness guru Simmons is an enduring phenomenon; his latest book, Farewell to Fat (GT Pub., 1996) has sold more than 600,000 copies. Here he offers 'reduced-fat' (not always 'low-fat') versions of his favorite desserts, from Keep on Truckin' Brownies to Yes, We Have a Banana Cream Pie to Some Like Hot Chocolate Souffl s, with color photographs of each one. Expect some demand.-