The nation's publishers, editors and booksellers are descending on Washington today for their annual convention, probably for the last time. The U.S. book industry once was so small and tightly knit that it could meet every year in the basement of the Shoreham Hotel. But now it sends more than 20,000 people to the annual convention and trade show of the American Booksellers Association, and space requirements for more than 1,300 exhibitors (book publishers, mostly) are such that the Washington Convention Center, six years after it opened, is already inadequate. This year's conventioneers will find not just close quarters at the Convention Center tomorrow but closely guarded ones. The death threats leveled against novelist Salman Rushdie and his publishers at Viking Penguin Inc. last winter persuaded the ABA to tighten access and security at the convention. The Fairfax Group, a Falls Church firm that provided security for NBC at the Seoul Olympics last year, has installed metal detectors at the entrances to the exhibit halls and will deploy plainclothes protective details and bomb-sniffing German shepherds around the convention floors. Fairfax President Michael Hershman refused to be more specific about the weekend precautions: "I don't want to give a road map to the other side." Oddly, apart from the security, the cause ce'le`bre of the bookselling world has all but vanished from the agenda -- and (except in a general discussion of censorship) is nowhere to be found on the schedule of panels for the ABA convention. "Having a debate just to have a debate seemed like a nonconstructive thing to do," said Bernard Rath, executive director of the ABA. "It's just preaching to the converted anyway, and potentially it could inflame the situation." So it seems that this year -- in truth, like every year -- it will be business as usual. This means four days of heroic wandering up and down the indoor avenues where publishers tout their autumn and winter offerings to bookstore owners, foreign-rights representatives, book reviewers and others in the trade, their competitors not least among them. The convention is not open to the public. Publishers huge and tiny, mainstream and off-the-wall, are represented here, side by side in booths under the white lights and struggling air conditioners. So are all manner of "sidelines" and other bookselling adjuncts and accessories, from greeting cards to books-on-tape to modular shelving. There is always talk about which book of the fall season will be the "hot" one, and some veterans claim that bestsellers can be sired by the right buzz at the ABA. Big-league publishers are pulling out the usual stops for their favored authors, with expensive parties around town, appearances at press conferences, readings, floor events and book-and-author breakfasts. Among the fall authors being so feted, to one degree or another, are Nancy Reagan, Umberto Eco, Robert Bork, Roy Blount Jr., Tracy Kidder, Susan Sontag, Tony Randall, E.D. Hirsch, Norman Cousins, Joe Clark, Reynolds Price, Bharati Mukherjee, Garry Trudeau, Pearl Bailey, Ken Follett, Kaye Gibbons, Robert B. Parker, Armistead Maupin, Russell Baker, Pete Dexter, Edwin Meese and Roseanne Barr. But in an industry where publishing conglomerates and vast bookstore chains are considered the norm if not the harbinger of doom, what is most striking is the astonishing diversity of those who attend the ABA convention every year -- and their pronounced smallness. Beyond buzz, baloney and blockbusters a more common and basic form of intercourse: The independent publisher, who lovingly brings out three or five books a season, comes to the convention and meets the mom-and-pop bookseller who knows just which customers will want to buy them. As the convention moves to Las Vegas (1990), New York (1991), Anaheim, Calif., (1992) and beyond, Washington is likely to be missed. According to Rath, the ABA's board of directors has decided not to book its annual convention, after 1992, in any facility with less than 600,000 square feet of exhibit space. The booksellers are here only because the ABA was desperate after Chicago reneged on its agreement to host the 1989 convention. But Washington's 381,000 square feet, Rath said, forced him to decline applications from more than 100 would-be exhibitors. A Convention Center official yesterday called Rath's gloomy assessment of a return to Washington "a huge disappointment" -- and one likely to be repeated with other major conventions until the facility can expand. Conditions inside cavernous convention facilities are an annual sore subject with ABA-goers. The last time the booksellers met in Washington, in 1987, Simon and Schuster was so incensed at being relegated to an inferior space (in a lower hall at the Convention Center) that it printed buttons that read "I the American Book Cellar Convention." Even so, Washington has been a sentimental favorite of ABA's repeat attenders. It not only is the original home of the event -- there's even a special "dinosaurs" gathering this weekend for those who remember the Shoreham days -- but it's exceptionally convenient to New York, where most major publishers are headquartered, and affords an unparalleled selection of party sites. This year, metro taxicabs will be disgorging hordes of sweat-soaked, name-tagged passengers at the doors of the Phillips Collection, the Library of Congress, the American Institute of Architects, the Woodrow Wilson House, the National Press Club, Protocol House, Prospect House, the Carnegie Library, the National Aquarium, the Folger Shakespeare Library, the National Museum of Women in the Arts, the Embassy of Mexico and -- dancing shoes, please -- Dakota.