5-Year-Old Southern Novel Enjoys a Sales Boom By Edwin McDowell [ A review of Masters of Atlantis, by Charles Portis, Transcribed by Alex T. Moore from The New York Times (April 9, 1984), for non-commercial use on The Unofficial Charles Portis Website (http://charlesportis.cjb.net). ] You won't find ''The Dog of the South'' on any best-seller list. What's more, you are unlikely to find Charles Portis's five-year-old novel in any bookstore, North or South - with the exception of the Madison Avenue Bookshop on Manhattan's Upper East Side. There the novel recently has been enjoying the sort of sales boom associated with the latest offering from Robert Ludlum, James Michener or Stephen King, and the publicity fallout has resulted in talk about a possible sale to a paperback publisher and the movies. One of the most bemused bystanders is Lynn Nesbit, the literary agent for Mr. Portis, a former foreign correspondent for The New York Herald Tribune. ''I was walking down Madison Avenue two weeks ago when I looked in the bookshop window and found it filled with 'The Dog of the South,' '' Miss Nesbit said. ''I was absolutely shocked. So I went in and said I was the agent and asked what had happened.'' 'Offbeat Southern Work' What had happened was that several employees of the 10-year-old bookshop, between 69th and 70th Streets, who have been fans of the novel since it was published in 1979, had the opportunity to show just how much they liked it. ''Kurt Thometz, one of our salesmen, turned me on to the book and the two of us have long sold it to people who like oddball, offbeat Southern work,'' said Gary Goldsmith, a buyer for the store. ''Even after it went out of print a few years ago, we'd pick up the odd copy in out-of-print shops and sell it.'' The novel tells of how one Raymond E. Midge of Little Rock, Ark., follows his wife to British Honduras (now Belize) after she runs off with another man, taking with her his Ford Torino and his tape-recording of a lecture by the University of Mississippi's Dr. Buddy Casey on the Siege of Vicksburg. Neither Mr. Goldsmith nor Mr. Thometz is a Southerner, but both like what Mr. Goldsmith described as the author's dry, deadpan humor. ''The book doesn't have a false note in it,'' he said. ''It becomes so clear why Raymond Midge's wife left him.'' Some weeks ago an employee of the bookshop noticed that the Random House catalogue listed ''The Dog of the South'' in its backlist. (The novel is published by Alfred A. Knopf, a subsidiary of Random House.) When the publisher confirmed that it had 183 copies in stock, Mr. Goldsmith ordered 25. 'They'll All Be Gone Soon' At the same time, however, the bookshop staff was preparing to change the window display, as it does every six weeks, and wondering what to feature. The shop's windows, which are designed by Gene Moore who also does Tiffany's windows, are often built around a theme, but sometimes they also feature an individual title. ''After I ordered the 25 copies, we started talking about putting 'The Dog of the South' in the window,'' Mr. Goldsmith said, ''and we were all so enthusiastic I called back and said we'll take all 183.'' It proved to be a wise choice, because all the store copies have sold out and now the shop will have to sell those in the window. ''They'll all be gone soon, then I don't know what we'll do,'' he lamented. But relief may be in sight. Gary Fisketjon, an editor at Random House, said that he is considering buying the book for the new Vintage Contemporaries, a fiction series of trade paperbacks that he will edit. ''I'll either publish that or 'Norwood,' '' he said, referring to another of Mr. Portis's books. Meanwhile, ''The Dog of the South'' continues to generate sales and comment. ''A lot of newsmen come in and ask, 'What are you doing with Charles Portis's book in the window?' '' Mr. Goldsmith said. From his home in Little Rock, Mr. Portis declared himself, ''surprised and very pleased'' by all the attention. ''Somebody mentioned a revival,'' he said. ''But that suggests a previous life, which the book didn't have much of.'' Mr. Portis, an Arkansas native who had been London bureau chief for The Herald Tribune, quit the newspaper in 1964 to try his hand at fiction. After winning a polite reception for his 1966 novel, ''Norwood,'' he hit pay dirt with ''True Grit,'' the story of 14- year-old Mattie Ross from Yell County, Ark., who enlists Rooster Cogburn to help track down her father's murderer. The motion picture made from the novel earned John Wayne an Oscar for his role as Cogburn, the meanest United States marshal in the nation. Mr. Portis is currently at work on a comic novel about a cult group, typing it in the same two-finger style he used to bang out his newspaper articles. ''I write in a little office without a phone, behind a beer joint called Cash McCoo's,'' he said. ''For $85 a month, you can't beat it.''