The People's Book Of Medical Tests. By Dr. David S. Sobel and Dr. Tom Ferguson. Summit Books. $12.95.

Medicine remains the last great consumer frontier. Physicians have, until quite recently, been able to stave off information-seeking patients with reassuring phrases and, occasionally, disapproving glances.

But just as General Motors and AT&T have learned that the consumer movement is likely to have a shattering and long-term impact, so too the healing arts are slowly being divested of their mysteries, with effects yet to be measured.

Patients are now asking for more information about drugs, specialists, assorted therapies and, of course, cost. And, keeping pace with the rising demand for data, a sophisticated arsenal of guidebooks is appearing on bookshelves around the country.

"The People's Book of Medical Tests" is a well-organized and thoughtful source of this kind of information. The material covered is sufficient to carry a prospective patient from the most minor skin tests for allergy to invasive techniques used to diagnose serious cardiac ailments or tumors.

Neither Sobel nor Ferguson is a novice in the guidebook business. Between them, these doctors have already contributed such works as "An Everyday Guide to Your Health" and "Medical Self-Care: Access to Health Tools" to the home medical library.

Armed with the "Medical Tests" book, a patient can understand the symptoms that led the physician to suspect a particular disease. Then the patient can prepare for the test to be performed and understand the procedure, how it feels and its risks.

In their preface to "Medical Tests," Sobel and Ferguson explain that their own and friends' experience led them to believe that "there was a tragic lack of information and support for people undergoing diagnostic tests, and a real need for a guide through the often frightening, sometimes dangerous world of medical testing."

In the introduction, Joe Graedon, author of "The People's Pharmacy," suggests the area of diagnostic testing has "remained largely inaccessible to consumers, up to now." This book not only sheds light on the testing arena but also gives instructions for 20-odd tests which can now be safely performed without professional help.

Although "The People's Book of Medical Tests" certainly supplies valuable facts for prospective tests, it does little to convince one that the best alternative for medical consumers is not effective communication with the physician.

Surely, if doctors spent more time explaining procedures, patients might not need to bone up with laymen's guides to highly technical and highly expert diagnostic exercises. And at what point will Drs. Sobel and Ferguson, and others, meaning well, dissuade some confused patient from some highly necessary test?

If as much energy were devoted to encouraging doctors to communicate directly with patients as in producing guidebooks, both parties might benefit.

In the meantime, "The People's Book of Medical Tests" is probably a good addition to the home medical shelves.