The book-publishing industry, beset by towering difficulties and responding with a Babel of remedies, is "on the threshold of major, dramatic changes," according to Townsend Hoopes, president of the Association of American Publishers. Among the future shocks: a radical change on bookstore racks as trade paperbacks crowd out hard-covers, $10 books sold in supermarkets and other nontraditional locations, and the rapid rise of electronic reading.

But by the time the AAP's midwinter meeting ended here yesterday, the problems were still more visible than the solutions. During three days of panel discussions and impassioned cocktailing at the Washington Hilton, most of the talk among the 450 representatives of the nation's major publishers concerned sluggish sales and thinning profit margins.

Various groups debated the effects of word processing and electronic textbooks. "Whether you like it or not, there's a certain inexorability about it," Hoopes said, predicting the advent in the '80s of computerized encyclopedias and professional reference books. Eventually books may be condensed into micro-chips so that 15 or 20 books can be mailed in a small envelope.

Textbook publishers especially complained of the Reagan administration's cuts in education funding, and Hoopes told members to urge their congressmen to stop President Reagan's "strangely regressive" economic program, which has caused "strain and possibly impending crisis" for the publishing industry. In addition to rising postal rates, copyright law and freedom of information issues, Hoopes warned of the effect of the budget deficits which "will hit the bond market like a 90,000-pound gorilla, crowding out anybody else."

Even in the worst of times, however, "there's something recession-proof about the book industry," Hoopes conceded. "It's like movies and liquor." And so it seemed at Wednesday night's opening reception, as executives rubbed worsted shoulders and rattled the ice cubes in their Scotches. "You all look like you're from Connecticut," said comedian Mark Russell. "The people up there are so preppy that the state bird is an alligator." Russell--the first in a lineup of guest speakers, including Joan Manley of Time Inc., Jonathan Yardley of The Washington Post and Hedrick Smith of The New York Times--was a thundering hit with jokes like: "I read the other day that one out of five adult Americans is illiterate . . . and that doesn't include the Moral Majority. Their moral is: If you can't read 'em, by God ban 'em!"

But censorship was a minor issue compared with the heated debate in the General Publishing and Mass-Market Paperback division over the future of trade paperbacks, which are "seen simultaneously as a revelation and a kind of a curse," said Peter Minichiello of Pocket Books.

The revelation is that trade paperbacks--the oversized volumes with heavier, more durable covers and list prices around $7.95--are the fastest growing segment of the industry. Spurred by such surprise best sellers as "The Joy of Sex" and Kathleen Woodiwiss' "Shanna," sales of such paperbacks rose from 1 million in 1971 to 44 million in 1980. Last year they accounted for as much as half the volume in some stores, and they now sell as well as hard-covers at B. Dalton, according to the chain's John Schulz, who predicts that they will overtake hardcover sales by 1983. "The generation that was weaned on paperbacks is now middle-aged," says Allan Kellock of the Waldenbooks chain, which is moving trade paperbacks into prime sales locations.

Change can't come soon enough for booksellers, who have been ailing for the past four years. In 1981, because of flat sales and higher shipping and energy costs, profits were down about 3 percent, according to a year-end financial profile compiled by the American Booksellers Association. Many retailers were forced out of business while others filled their shelves with posters, book bags, games and T-shirts.

At the same time, new markets are opening. Since only about 25 percent of the American public buys books, said David Youngstrom, a wholesaler from Denver, "we have to find new people to sell to." His firm is doing so by supplying hard-cover and trade paperbacks to chain supermarkets in Colorado, with remarkable success: Many stores now have eight-foot bookracks and some have 70-foot "book sections" with mostly trade paperbacks. And libraries are changing. James O'Neill of the D.C. Library system told the publishers that, with rising hard-cover costs and reduced funding, libraries have "little choice except to order the paperback edition if it is available."

But the curse is that the industry still doesn't know what to make of the boom. Customers are unsure of the difference between mass-market paperbacks and their trade counterparts. Booksellers have their own problems. ABA president Joan M. Ripley advised publishers to provide more "dignified" covers, faster delivery, standardized sizes to avoid shelf-display problems and a product with "the look and feel of a real book."

And publishers themselves are uncertain how to exploit the trend or what categories of books will succeed in the trade-paperback format. In a mock publishing decision session considering six hypothetical titles, after lengthy war-gaming only two were deemed economically feasible. One was the "Better Homemakers Cookbook," which at 6-by-9 inches with artwork and a production cost of 97 cents could be published at $8.95 to $9.95, a $5 savings from the hypothetical hard-cover price. The other volume considered successful was a consumer study of fast-food nutrition, felt to be a natural for the trade-paperback audience. Titles rejected as infeasible included an expensive home-decorating book with many color plates (the audience would pay extra for the hard-cover) and three novels: a solid literary work with a movie sale (consigned to hard-cover); a family/historical saga called "The Passionate Pilgrims" (less risky as a mass-market paperback) and a multi-generational romance called "The Doctor's Daughter," about "three generations of Jewish female doctors," one of whom was FDR's secret mistress (a possible blockbuster, it should start as a hard-cover for the best reprint deal.)

The trade-paper prognosis for fiction is poor, despite the recent simultaneous hard-cover publication of novels by Jerzy Kosinski and Thomas McGuane. Skeptics regard the dual-format trend as simply a way of keeping hard-cover prices artificially high to make the trade-paper edition appear attractive by contrast. "Hard-cover publishers are just fooling around with trade paper to get around price resistance," said Stanley Reisner, president of Pinnacle Books.

Another disincentive for fiction: Critics rarely review trade paperbacks. That issue arose at the Thursday luncheon meeting when Yardley's address turned into what Hoopes called "a donnybrook." Although he called trade paperbacks "the wave of the future," Yardley read the current trade-paperback best-seller list (with such titles as "Mastering Rubik's Cube" and "Garfield Gains Weight") and said that "it's hard for anyone who loves literature these days to have much good to say about trade or mass-market paperbacks." This provoked a hot retort from Ron Busch, outspoken president of Pocket Books, who argued that periodicals are "not fulfilling their obligation to the public" by omitting reviews of trade and mass-market paperbacks.

That exchange was virtually the only mention of literary quality during the three-day drawing of bottom lines. Maxwell Perkins, someone suggested at a coffee break, might not have survived in today's desperate financial climate. "Well, Maxwell Perkins didn't make much money for the house," said an official from one publishing house. "That's why, after 40 years, he died an editor!"