The subtitle, ""Rest Begins At Forty"", is a better catch title. The actual title is more definitive, more challenging to the accepted tenets of our national faith in the god of exercise. Dr. Steincrohn's aim is to prove beyond doubt that overexertion is unnecessary for happiness and health, and that for those over 40 it is detrimental, a gamble at high stakes. He debunks most popular diets -- bases weight reduction on calories alone; he studies the physiology of muscle and nerve behaviour to prove the fatality of excess exercise; he contends that relaxation is of more value when one is not over-fatigued; he parallels the pattern of woman and her work, man and his, and the causes of fatigue, need for rest. Lots of good commonsense, but not a popular thesis.