The character of the American bookstore, which has changed significantly in the past quarter-century, seems on the verge of undergoing another major transformation: It is becoming a place that sells not merely material to be read, but also material to be listened to or watched. This is probably not so dire a development as traditionalists may imagine it to be, since anything that brings customers into bookstores is likely to be good for books in one way or another, but it does have the potential to muddy still further the distinction between books and adaptations of them.

In the years since World War II, the bookstore has become a very different place from the cozy literary warren that the nostalgic would like it to be. Booksellers, faced with steadily increased competition from other merchants, have filled their stores with new wares and with ones previously sold through other outlets. The relatively inexpensive paperback has eclipsed the hardcover book, and in most stores commands the overwhelming preponderance of shelf space. In addition to books, many bookstores now sell greeting cards, calendars, puzzles and games, computer software and diaries.

These products may not have much to do with books, but they do have something to do with reading and writing. The ones now making their way into the store, by contrast, have a less clear relationship with books. These are tapes, both audio and video, and there can be little question that they will play an important role in the bookstore of the very near future. Their connection with books and reading ranges from the immediate to the tenuous, but they do not seem particularly out of place in the bookstore and they certainly should increase its owners' expectation of profit -- though whether at the price of attention and space devoted to books remains to be seen.

The connection between audio tapes and books needs no elaboration. The overwhelming majority of audio tapes sold in bookstores are recordings of books, ranging from 19th-century classics to contemporary thrillers. They are bought by people who use them to squeeze in "reading" time while driving, jogging or otherwise occupied. Though listening to books does not seem to me as satisfying as reading (and thus interpreting) them oneself, this is a matter of personal taste; the only thing that stands between the listener and the author of the book is the voice of the reader, which certainly is a relatively slight intrusion on the author-reader relationship. Recorded books are advantageous for many readers and good for many authors, for whom they are a new and welcome source of income.

Videotapes, though, are a more complex and ambiguous proposition, because their connection with books involves a major intrusion on this same reader-author relationship. But there seems little doubt that however modest their presence in the bookstores may now be, it is here to stay and will only grow larger. This is because, as was demonstrated at the annual convention of the Video Software Dealers' Association in Washington last month, videotape manufacturers and distributors see the bookstore as a more promising outlet for videotapes than, remarkably enough, the videotape store.

As was reported in Publishers Weekly recently, "video stores, to date, have posted a sales record that tape publishers find disappointing." One person said that "a lot of times, people walk into a video store and don't think it's anything but rental," while another stated her case in retailing jargon: "It will be a tough haul to convert video stores to a sales orientation. The video store is in a rental mode, and it's tough to go against the grain."

Enter the bookstore. As the same woman put it, "We are making an aggressive attempt to get into bookstores," for the simple reason that "bookstores sell products; video stores rent them." One representative of a video distributor said: "It's naive for booksellers to think video is not important in an entertainment-information business. Competition for leisure-time dollars is much more intense than a few years ago. And bookstores should consider: what are they going to replace discount erosion with? I'm not saying that video is right for everyone at this time -- it might be too new. But bookstores don't have to change a total mentality to handle video."

He was right, as was the person who said that "bookstores have to change with the times." But though bookstores may have to change in order to meet the competition, it should not be necessary also to change our understanding of what a book is. This, unfortunately, the video manufacturers apparently would like to do. A campaign has recently gotten under way that is clearly designed to persuade us that to watch the film adaptation of a book is the same as, if not better than, to read the book itself.

One widely published advertisement, for MGM/UA, offers the promise of "GREAT BOOKS, GREAT GIFTS, GREAT ENTERTAINMENT." In smaller type it says: "Here come the Great Books -- now a video collection from MGM/UA. Watch your favorite characters come to life . . . Watched any great books lately?" Then, under a picture of some of the tapes, which are packaged to look like leather-bound books, it lists the "books" available at $24.95 apiece. Among them are "A Tale of Two Cities," "David Copperfield," "Mutiny on the Bounty," "Pride and Prejudice" and "The Red Badge of Courage."

As it happens several of these are excellent movies, and it would be a pleasure to have them in a private video collection. But they are not books, never will be books, and to promote them as the equivalent of books is a grave misrepresentation. The old Charles Laughton adaptation of "Mutiny on the Bounty" is a grand film, but it is not the book that Charles Nordhoff and James Hall wrote; the same is true of every other movie that MGM/UA is selling in this campaign, and of every other movie ever based on a book.

It really should not be necessary to elaborate the point, but a filmed adaptation of a book not merely condenses and distorts a book for dramatic convenience, it also completely severs the reader-author relationship. The author does not speak directly to the reader when his book is filmed; instead, the screenwriter, the director, the actors and the editor speak for him, often with little concern for maintaining the spirit or integrity of his words. Even when they do attempt to honor the author's intentions, the visual images they present rob him of the ability to create his own images in the reader's mind; what we see is theirs, not his.

The danger in this latest effort to equate movies with books is that readers, especially young ones, will be persuaded that if they "watch" a book they no longer have any reason to read it. To the contrary, the message being sent them by parents and teachers -- and manufacturers, too -- should be: Read the book. If you like it and want to watch the movie, by all means do. But the book is the book, the movie is the movie, and never the twain shall meet.