Finally a book that can serve the geriatric set the way Dr. Spock's "Baby and Child Care" soothed generations of first-time parents.

Dr. Richard Jed Wyatt, chief of the adult psychiatry branch at the National Institute of Mental Health, has provided a 347-page encyclopedia of ailments most likely to afflict those over 50. All of the body's major systems -- digestive, nervous, circulatory and respiratory -- get attention, and a detailed index lets readers flip to sections or even paragraphs that cover specific physical problems of personal interest.

This is not, however, what one would call a good summer read. Rather, it is a book to be consulted as the unexpected and often perplexing questions of aging inevitably arise. It probably will be most useful as a bedside consultant.

Written in a practical and human style, "After Middle Age" gives a broad-brush treatment to concerns as diverse as sex after major surgery or heart attack, senility, osteoporosis, tennis elbow, changes in diet, sleep and nutrition in the aging person, and the diagnosis, prevention and treatment of depression.

Perhaps the most useful chapters are those on choosing a physician, the guide to private or nursing home care and the sections on insurance and living independently. Wyatt urges the elderly to learn assertiveness so they will be better equipped to ask tough questions of physicians and family and demand to be treated like adults.

Wyatt sidesteps the question of how much exercise is necessary or good for those over 55 or which regimen is most likely to prolong healthy bodies with the fewest torn ligaments and sprung joints. He suggests that all newcomers to fitness fads take it slowly and cautiously.

"At 30, you might have wisely considered adding 5 minutes a day if you were playing tennis daily for the first time. After 55, I would seriously consider adding no more than 5 minutes a month. At this rate you will be able to play a full hour after a year, perhaps without ill effects."

Wyatt sees work or a retirement structured around definite interests as the best way to keep mentally agile, alert and in good spirits.

If read cover to cover or even several chapters at a time, "After Middle Age" is depressing -- something akin to being leveled by a sledgehammer that warns of all the failing systems that lie just around the bend.

But used sparingly by both seniors and their children, the book is an informative and detailed guide to aging.