Things had been a bit slow at the Washingtonian Book Festival on Sunday afternoon, what with the shamelessly stellar weather (which had even Washingtonian magazine publisher Larry Adler admitting he'd REALLY rather be playing tennis) and the American Booksellers Association annual convention (THE serious, mandatory, convivial event of the publishing year) just two weeks away. Most of the big publishing houses of New Yor and Boston were in the midst of sales conferences before announcing their new autumn lists at the ABA in San Francisco and just couldn't see their way to setting up another display in Washington.

But then a rather unusual, somewhat outrageous, little known, but thoroughly publicized author was announced, and 200 people tromped over to hear her. They were curious and excited. This was the sort of thing they had come for - what had been missing for most of the weekend in the chilly, fluorescent, subterranean hold of the Sherator Park Hotel.

Colleen McCullough, author of "The Thorn Birds" - the novel that (in case one has been visiting in Outer Mongolia these last weeks and missed the news) has been touted as the Australian "Gone With the Wind" and whose paperback rights were bought by Avon from Harper & Row for a record-breaking $1.9 million - livened things up a bit.

A self-described eccentric and "inappropriate millionairess," she told the audience she almost wished she hadn't made all that money because she doesn't quite know what to do with it.

The audience seemed quite taken by her, particularly when she laughed he characteristic belly-laugh, and said that a friend who was having dinner at her house in Cambridge the night she received a call from her editor about the paperback rights, had found himself "supporting 200 pounds of McCullough which was about 180 pounds more of McCullough than he had any right to expect."

McCullough was one of 70 authors featured at this, the Washingtonian's first book fest. Friday night saw a panel discussion on "The Washington Writer" with Richard Reeves. ("Convention," "A Ford, Not a Lincoln") Patrick Anderson ("The President's Mistress") and Abigail McCarthy ("Private Faces, Public Places" and "Circles: A Washington Story"), who expressed varying degrees of enthusiasm for Washington as litrary "material," but neatly avoided coming to blows on the subject.

There was an author specializing in something to suit most anyone's taste, from Esquire's Roy Andries de Groot on Cuisinart cookery to former Look magazine editor J. Robert Moskin on his latest, "The U.S. Marine Corps Story" (a handy volume of 1.039 pages): and from Allan C. Hodges and James M. Goode of the Smithsonian on the respective delights of walking tours and outdoor sculpture to former CIA deputy director Ray S. Cline on, as one might expect, "Secrets, Spies, and Scholars." There was even a young lawyer in tortoise-rimmed glasses who read his own poetry, a continuing calligraphy demonstration by Dick Jackson, who would pen - exquisitely - whatever one wanted, a "Stump the Librarian" contest held by public libraries in the area (who were able to answer the old saw about the final resting place of Mr - and Mrs. - Ulysses S. Grant), and ongoing chess games sponsored by the Maryland Chess Association a quiet oasis in a very ah, verbal room.

Some of the more venerable, or at least better known, publishing houses were represented - Harper & Row and Avon, of course, as well as Doubleday, Dial/Delacorte/Delta, McGraw-Hill, Time/Life, and various university presses. And then there was a display from David R. Godine of Boston, whose art books and miniature "Chapbooks" of poetry are among the handsomest books around, and a demonstration of bookbinding - the proverbial slim volumes beautifully turned out by a group from Alexandria.

"We did very well with small, local publishers," said Larry Adler, who estimated attendance at something short of 5,000 by Sunday afternoon. "I can tell you we'll lose money," he said, a bit regretfully. The mistake was in the timing - late spring weather, the impending ABA. In choosing dates, he and the magazine staff had been more concerned with what Washington events might or might not interfere with the Festival; Adler is thinking tentatively in terms of next fall for the second annual Washingtonian Book Festival, if the local bibliophiles are interested. "Well, what can I tell you" he said. "We've learned a lot. It's been a good investment."