The Changing Desert, by Ada and Frank Graham. San Francisco: Sierra Club Books (distributed by Charles Scribner's Sons, New York.) 90 pp. $11.95.

This is a thoughtful introduction to the natural history of the deserts of our American Southwest. The Grahams tell us how these deserts came to be and what makes them so fragile and special.

Readers are introduced to the saguaro cactus, the soapweed, and the bighorn sheep. Youngsters who are used to cattle rustlers on television will meet cactus rustlers in the Arizona desert. Others who dream of racing motorcycles across ''waste'' land will pause as they consider what their bikes would destroy. The Grahams seem to be saying, you can't help but appreciate the desert, if you know it.

This Sierra Club book, with excellent illustrations by Edward Shetterly, is directed toward teaching future adults to defend endangered natural areas.