Thanks to the national fitness boom, stores are full of health and beauty books--their jackets covered with lithe and rippling bodies, their promises miraculous, their sales astronomical. (Jane Fonda's Workout Book has gone through 22 printings to date.)

But learning to exercise from a book is difficult. The reader must have firm self-discipline, and the author must have real talent, not just a niche on prime-time television or a portfolio of glamorous photos. No one ever got fit by osmosis. In choosing a book, first determine your specific goal: enhancing your cosmetic appeal, beginning an exercise program or upgrading your current one, losing weight or improving your heart-lung performance. Then look for pictures or diagrams with clear explanations, practical advice rather than vague sentiments and personal anecdotes, and authors with real expertise or credentials.

Most volumes fall into three categories--beauty counseling, active exercise and "soft" exercise or stretching. The best of the first group is Linda Evans Beauty and Exercise Book (Simon & Schuster, paperback, $9.95). Evans believes that "outer beauty is a direct result of what we feel inside," and the first half of the book deals with inner resources, including effective relaxation techniques such as "visualization" (closing your eyes and imagining the look and feel of an object) and a sensory exercise called "intuitive awareness." The second half gives specific tips on care of hair (get rid of split ends by braiding your hair, brushing upward and snipping off the frizzies), nails (use a tea bag, coffee filter and Duco cement), and skin (for "facial rejuvenation," press the carotid artery and massage).

Her exercise section, however, is less useful to the average reader. Evans has thin legs, broad shoulders, a large bust and narrow hips, and most routines are tailored to her shape. One expects the worst from TV-star books. But Evans, who stars in Dynasty, is inspiring in her positive attitude and personable approach.

Unless you're a permanent member of the Coppertone set, Christie Brinkley's Outdoor Beauty & Fitness Book (Simon and Schuster, $16.95) may not be useful. It's all about how to be beautiful while romping in the sun (and afterward), and valuable for choosing the most effective sunscreen for your skin, the most appealing hats and sunglasses for your looks and the best bathing suit for your figure. There is also advice on diet, a small section on exercise and far too many pretty but useless pictures of Brinkley in various outfits.

In the active-exercise category, Jacki Sorensen's Aerobic Lifestyle Book (with Bill Bruns, Poseidon, $15.95) tops the list, especially for more experienced exercisers. Sorensen is the pioneer of aerobic dance, and if you want to learn about aerobic fitness, this well-researched and thoroughly informative book is the place. There are four full chapters devoted to toning and dance routines (with recommended music and such clear step-by-step pictures that whole dance sequences can be learned) and a wealth of material on oxygen consumption, heart rate monitoring, injury prevention-- even the right shoes for a variety of sports.

There are also cardio-vascular fitness alternatives for those limited by injury or age, a long and splendid section on how to choose an aerobic class, and a final question- and-answer session with a noted exercise physiologist which clears up much popular misinformation. On the "myth" of "spot reducing": "It's possible to do spot exercises which will work on isolated muscles, but spot reducing is simply not possible . . . fat is fat and is not burned by the body from specific places. Instead, the only thing that will help is regular aerobic exercise and a healthy diet."

For fitness beginners, The I Love N.Y. Fitness Book by Suzy Chaffee and Bill Adler (Morrow, $17.95) is a practical catalogue of safe and time-proven exercises. As in Sorenson's book, pictures and captions are immediately informative--especially in the sections on posture, exercises to do while waiting and "exercises with the man in your life." But sometimes Chaffee stretches too far: The opening group of aerobic warm-up routines could possibly be dangerous to some readers. She suggests jogging in place, jumping jacks, high leg kicks and other ballistic movements as "preliminary exercises . . . to warm up your body by speeding up the action of your heart and lungs and gently limbering up your muscles." These will get the blood moving, but will not properly warm up the groups of muscles surrounding the joints. Avoid risk by simply using her "stretch warm-ups" both before and after the aerobic warm-ups. In addition, the language ("gorgeous" this, "pretty" that) is terminally cute at times, and there's some fat at the end: A largely useless section on "Super Mom" exercises. Still, it's a good basic manual for folks who want to "go it alone."

By contrast, in The Body Principal, (Simon and Schuster, $16.95) Dallas' Victoria Principal promises a shortcut to fitness by doing isometric toning exercises throughout your daily routine: in the car, while showering, during a manicure. Each provides an opportunity for more fan-magazine photos of Principal, often so self-consciously sexy or irrelevant that they are actually distracting to those trying to learn. She emphasizes resistance exercises that can be done anywhere. Some are useful (push-ups against the bathroom counter): many are ridiculous (abdominal leg-lifts performed on the bed, which provides no support for the lower back). Moreover, nearly a quarter of the book is devoted to explaining the use of the Universal Gym Machine--information widely available for free on wall charts accompanying the equipment.

Finally, Stephanie Sorine offers an unusual exercise program in The French Riviera Body Book, (with photographs by Daniel S. Sorine, St. Martin's, $13.95)--a series of stretching movements somewhere between ballet and tai chi. Of interest mainly to dancers, the book is often long-winded and tedious. The anatomical information is correct, but needlessly detailed and wordy, not suitable for an exercise manual.

In the last category--soft exercise and stretching-- two books stand out. Soft Exercise: The Complete Book of Stretching, by Arthur Balaskas and John Stirk (Scribner paperback, $9.95) clearly explains dozens of yoga-like stretching routines and is also an excellent guide to the neuromuscular system. Some of the routines are innovative (for example, a torso stretch done against the wall) and specific stretches recommended for individual sports. But be careful. Some of them--involving hyperextension and excessive rotation of the joints-- could cause extreme stress if not done carefully.

The YWCA Way to Physical Fitness: How to Make What You Have Better, by Evelyn Fiore with consultant Elaine Quinn (Doubleday, paperback, $9.95) is a splendid reference for anatomical review. The opening section explains the mechanics of the spinal column, muscular system, cardio-vascular system and their interrelationship. The second part offers a well illustrated series of "safe" exercises drawn from a national survey of the Y's programs, each accompanied with specific cautionary notes. The final section is devoted to the "working back," focusing on alignment techniques, constructive relaxation, neck and back stretches to relieve tension, and the importance of breathing while stretching.