Phil Potter, 34, conventional, one of the short-haired breed -- you might meet him anywhere with a glass in his hand -- leaves Jessica, his wife of a few decelerating years, and a P.R. job to start over in Boston at a small unheard of junior college where he gives a seminar in Communications. He also has all the options of being free -- which comes down to an evening alone of T.V. dinners and T.V. movies submerged in Scotch. But in time he meets Marilyn who is ne plus alluring and he becomes one of her activities (pottery, a course in Existentialism, her shrink); in fact they are ""an item"" until the bloom is off the sex in spite of desperate attempts to revive it -- a weekend at a Vermont inn (New England boiled dinner and Indian pudding) or the Sylvan Swirl from The Sensuous Woman. After that they decide to remain friends doing things together and advising each other (she has an affair with a married man; he finds himself indulging in random, obliging sex). But the ceiling gets lower and lower in this world where everybody is either married and miserable, or divorced and miserable, and the whole book is undershot with the desolate idea that nothing lasts while, worse, the need seems to go on forever. En route -- whether it's a commune or a gathering at the Harvard Club or a session of Assertion Group Therapy -- the scenery is varied and mobile and the contemporary artifacts are letter perfect. . . . An astute and funny book, a sad and funny book, a sophisticated and funny book -- one of those seldom pleasures.