Paper moons and town parades: slices of life from an America gone by; Prairie Fires and Paper Moons: The American Photographic Postcard, 1900-1920, by Hal Morgan and Andreas Brown. Boston: David R. Godine. $25.

Picture post cards, as we know them today, are usually limited to glossy, high-colored views of the Grand Canyon, resort hotels, and other attractions along the tourist route. But the sepia-tinted photographic post cards of the early 20th century depicted a much broader, usually far more personal, range of scenes - everything from a local flood to a family portrait to the man in the moon.

The reason is that photographic post cards, which were in their heyday from 1898 until about 1920, were one of the chief ways that friends and relatives living long distances apart kept in touch. Photographic studios across the country turned portraits - often taken against fantastically engineered settings - into post cards that could be mailed for a penny. If you didn't want your own portrait on a card, it could just as easily be one of the family dog or the circus parade that marched through town last week.

All of these everyday subjects, plus a few extraordinary ones, are documented in this handsome new book. While gathering their material, authors Hal Morgan and Andreas Brown talked with dozens of collectors and searched through attics across the nation, unearthing boxes of photographic post cards. Much of what they found stands as strangely affecting records of middle-class Americans at the turn of the century.

Why Morgan and Brown chose to call their book ''Prairie Fires and Paper Moons'' is apparent after even a quick glance through the plentiful illustrations. Paper moons were among the wildly imaginative sets that photographers used to pose their subjects against. The book's cover illustration is perhaps the prime example: A bearded gentleman sits on a quarter moon crescent while holding an American flag. As a final touch of whimsy, his hat hangs on the top tip of the crescent.

There is also a post card depicting a prairie fire that blazed through Tripp County, S.D., in 1909. Others are of barn fires, floods, tornadoes, and the huge hailstones collected after a record-breaking storm fell on Ostrander, Minn., in 1908. The authors explain that such natural disasters became post card subjects primarily because it was the only way the public could have a visual record of them. Most small-town newspapers at the time did not yet use photographs to illustrate the local news covered in their columns.

But most of the photographic post cards that people sent to each other featured ''news'' of a far less dramatic nature. Many were simply portraits of the sender or of the sender's children, quick and easy ways to let grandparents or old friends know how the family was getting along. It could be said that the photograph post card was used in much the same way that the long-distance telephone call is today.

What is most eye-catching about many of the portrait cards in the book is the great effort photographers made to place ordinary people in extraordinary settings. Besides the popular paper moon motif, studio sets showed subjects riding atop Halley's Comet or swimming through the rapids at Niagara Falls. Many such post cards were the specialties of photography studios at amusement parks and seaside resorts, humorous mementos of a holiday trip.

The workplace was also a frequently used setting for photographic post cards, and these are some of the most historically interesting examples in the book. There are icemen and their wagons, telephone operators at their switchboards, barrelmakers, and crews working on the railroad. Some of the cards are vivid records of the early labor movement, such as one of machinists in Baltimore on strike for the revolutionary demand of an eight-hour day.

Soldiers fighting in World War I used the photographic post card to send news of themselves and a very personal view of the front lines to family and friends back home. There is a post card from a young sailor photographed among the mess crew on board the USS Louisiana, and one of a solitary soldier who lists the location of his portrait simply as ''somewhere in France.''

After World War I photographic post cards, like the world at large, became more sophisticated and far less personal. Highly prized by collectors, the early cards are intimate slices of life from an America that can never be photographed again.