From the retired chief neurosurgeon at Boston City Hospital, aided by the author of The Empire Builders (1987), an expert's nontechnical but not simplistic survey of brain problems and their prevention. Early on comes a preliminary mental-status test that Dr. Mark gave to nursing-home patients and that readers can give themselves or administer to ""loved ones"" they're concerned about. Reviewing causes of mental loss, the authors assure readers that ""benign memory loss"" (e.g., misplacing car keys) is common after 45 and not progressive or serious; Alzheimer's disease, on the other hand, is progressive and currently incurable, but often complicated by reversible conditions or feared when the real problem is depression or some other treatable condition. The rest of the book is Dr. Mark's brain-power program, a regimen similar to those for physical fitness in that it's made up of a nutrient-rich, low-fat diet (complete with two-week menu) and regular exercise (in this case, mental as well as physical). Dr. Mark departs from the conventional wisdom of the past in ""mega-nutrient"" doses, and from conventional discretion in recommending the products of two companies with whom he has ""made arrangements"" to put out his formula. But the recommendations are well supported by research and experience, and he avoids the exaggerated claims and enthusiastic come-ons so common to the genre.