GENERALLY SPEAKING, there are two kinds of paperback books in America: mass-market paperbacks and trade paperbacks. How do you tell the difference?

It's not obvious. The difference has nothing to do with whether a book is big or little, tall or fat or pocket-sized, fiction or nonfiction, a best-seller or an abstruse treatise on philology. You can't count on telling the difference by picking the book up to feel the paper, and, especially now that you often need more than two bucks for any paperback, you can't go by price.

Quite simply, the only meaninful difference between a mass-market paperback and a trade paperback lies in its distribution. Mass-market paperbacks are handled by magazine distributors and are sold in drugstores, chain stores, airports, newstands, candy and cigar stores, as well as in bookstores. Trade paperbacks (the word "trade" refers to ordinary, commercial books as opposed to academic or specialized ones) are sold in exactly the same way hardcover books are sold, through bookstores.

Here is a cautious history of paperback books in this country, cautious because there are a number of exceptions to the rules, and, besides, in the last few years all the rules have changed. As you will see.

Mass-market paperbacks, cheap, easily portable reprints of popular hardcovers, came to America (Penguin had already started up in England) just before World War II. In the early days they looked different - maybe some readers can remember the old wartime books, wider than they were long. Soon, however, the present four-by-seven-inch (approximately) format evolved, and distributors began to offer book racks of standard sizes to match the magazine racks. Dell and Fawcett were part of successful magazine empires; Avon belonged to Hearst; Pocket Books (the first paperback house, and for a long time the best-known), Bantam and the others made arrangements with national magazine distributors. All of the publishing houses were cheerfully low-brow, catering to mass tastes by wrapping the most high-minded novels in lurid covers with come-hither copy ("What was the shameful secret that forced Felix Krull to confess: Only one woman knew . . .")

Then, in 1952, the trade paperback was invented by a kid right out of college. A rookie editor name Jason Epstein persuaded the top brass at Doubleday that studens of his generation would buy serious books if they could afford them - and that the way to reach them was to publish paperbacks to be sold in bookstores, especially college bookstores. These would have much smaller printings than the mass-market books, would be printed on better paper, bound better, and sold for a much higher price: perhaps 85 cents or 95 cents compared to the popular 35 cents to 50 cents.

The 12 books published in the first Doubleday Anchor line sold out immediately, and many more followed. The new strange-looking paperbacks with the Edward Gorey cover drawings made a hit not only with students and intellectuals, but with ordinary bookstore browsers looking for something new. And cheap.

Anchor and Knopf's almost identical Vintage Books prospered alone for a while, but gradually new ventures began to broaden the market.

Hardcover houses began to publish soft-cover versions of their own books, generally for school use. These were/are almost always printed in a larger "book" size so that the print would not have to be reset, or photographed and scaled down, to fit a smaller "pocket book" page. The covers were, to be kind, unimaginative.

Some mass paperback houses had already formed their own "classics" lines (New American Library's "Signet" is an example) for the school markets, and they now began to distribute them through the trade. Usually these featured standard authors like Shakespeare or Dickens who were also "public domain" - dead long enough so that copyright laws no longer applied to their work. So many different editions appeared of some books that companies began to develop newer, wilier marketing techniques especially for schools. This was hard on textbook manufacturers, but students loved it.

"Specialty" or "gift" books turned up, often in stationery stores. These came on odd shapes and sizes and no one knew quite what to make of them. Price, Stern, Sloane, one of the first of these publishers, designed their own odd-sized racks for their game and joke publications, and solved the definition problem temporarily by calling them "non-books" - a term that seemed a little lame after How to Be a Jewsih Mother became a best seller.

Next, Ian Ballantine, who had broken away from Bantam to start his own firm, brought out the first soft-cover picture book. Actually, picture books were not really brand-new, it's just that nobody had yet identified them as books - the Museums of Modern Art's beautiful collection of photographs, The Family of Man , and been selling briskly for years as a "catalog." But Ballantine, in conjunction with the Sierra Club, in 1967 published the first of their four-color "coffee-table" nature books, In Wildness Is the Preservation of the World , to great success. Other Sierra Club books followed, then a line of softcover art books.

Meanwhile, the countercultural revolution of the'60s was beginning to hit the book business. Tiny upstart publishing houses far from the New York establishment began putting together their own books, different in size, shape and content. Most of these were economically bound in paper. One made history by winning a National Book Award in 1972 though, predictably, many conservatives refused to admit it was a "book" at all. That winner was, of course, The Last Whole Earth Catalog .

If 1952 was the year the trade paperback industry was born, 1972 was surely the year it graduated. With the huge success of this unwidely compendium of mail-order items, odd facts and homespun lore, printed on rough paper in a variety of typefaces, publishers realized, as though light bulbs had winked on over their heads, that a book could be anything at all! Shape, bulk, price, content didn't matter. What you called it didn't matter - if a bookstore would sell it, it was a book. And if people would buy it, you got rich.

The rest is more or less history, but very busy history. Here are a few of the things that have happened:

Almost all larger mass-market houses now publishe trade paperbacks, some ocassionally, some in a regular seasonal line. Almost all larger hardcover houses publish trade paperbacks, some occasionally, some in a regular line (the occasional releases can be imaginative, as when Houghton Mifflin put out Even Cowgirls Get the Blues to catch the market that had welcomed Richard Brautigan and the Delta reissues of Kurt Vonnegut in trade paper). Some distributing their books through the sales facilities of larger companies and some, like Workaman Publishing (which does B. Kliban) establishing their own sales forces. There are hundreds of small presses operating in trade paper today: it is the medium of the independent entrepreneur.

How-tos! Guides! Plants, magic, tap-dancing, recipes, crafts, sex, pets, cartoons, movies . . . And catalogs!It seems as though every publisher in America has thrown itself into the catalog business - we have become a nation of list-makers. And the lists are now, of course, offically books. As are the Star Trek Blueprints . As are even posters, singly or bound together. As the trade-paper business boomed, even Vintage and Anchor reached out of their old formats to try some of the new ideas, a picture-book book on "hot tubs," for instance. The fascinating result of all this activity is that if you pick up a soft-bound book from a book shop table now, you simply cannot tell (without looking) if the parent publisher is hardcover, paperback, small-press of gift-item. Try it.

The latest adventure in trade-paper publishing is original fiction. Experimental fiction from small presses has had limited distribution, and mass-market houses have become increasinly daring with large printings on original novels, but trade paperbackers feared to lose both the library and review acceptance of hardcover and the impulse rack sales of mass paperback. But somebody had to try it, and this year Avon's romance, Shanna , and Ballantine's illustrated fantasy, The Sword of Shannara , have paid off in dollars. Now, crowding ahead, is Knopf's The Serial . This isn't exactly a novel, at least not coventionally, and it's spiral-bound! The old dilemma: what to call it? What about a trade paperback?

Nobody is baking funeral meats for the hardcover industry yet. Mass-market publishing is in its prime. But the youthful exuberance, flexibility and - perhaps most of all - unpredictability of the trade paperback business have convinced a lot of people that this is where the action is. Just wait and see.