Halfway through this extended complaint the authors get around to saying the obvious: ""the mistress, like the prostitute, fits uneasily into the feminist perspective."" So true. Of the forty women whose affairs with married men Kedgley and James investigated, most admit to feelings of guilt, resentment, jealousy, isolation and insecurity. ""I had a low--I mean really low--image of myself,"" says Beverly, looking back on her two-year affair with a married lover. Many vow they will never again repeat the experience after the difficult realization that ""I was putting all my energy into trying to please him. . . . I had no energy or time to think about my feelings."" Far from being marriage wreckers, the authors report that many, if not most, of the women they talked to ""were obviously playing a marriage-making role""--they provided a safety valve and the man's marriage collapsed once the affair was terminated. What's good about it? Well, the sex, mostly. And, if the woman is also married sometimes the extracurricular liaison will perk up her dull life with her husband. But by all reports the ""mystique"" of the mistress as a glamorous adventuress--a Mme. Pompadour or Emma Hamilton--is so much twaddle. On the whole, a depressing report on a masochistic lifestyle.