IF YOU can't sell cookbooks, you can't sell your mother a box of Christmas cards, or so say some booksellers in the area. Cookbooks at Christmas sell like aftershave on Father's Day. Out of the 15,000 or so choices, there must certainly be one for everybody.

"They're the bread and butter of the bookstore business," says Michael Sullivan, assistant manager of Reprint Bookshop in L'Enfant Plaza, a general bookstore that stocks 300 cookbook titles. Those devoted to chili are his hottest sellers. "Cookbooks are perennial classics, this time of year especially," said Sullivan.

So it was more than fortuitous that The Cookbook Store should open in late September at Georgetown Park Mall. A tiny shop, it is crammed to the brim with the obsession of compulsive recipe collectors -- regional cookbooks. Noting that the store carries about 3,000 titles, manager Teri Tobin says that these regional cookbooks are by far the best sellers.

Julia Child, in fact, takes a back shelf at The Cookbook Store to the Junior League of Rochester, N.Y., which has published "Applehood and Motherpie" ($12.95), and released it in a binder cover that bends to form its own bookstand. The pages lie flat and the cover is washable.

The top seller is a book called "Forum Feasts," a $7.50 book published by a home for the mentally handicapped in New Jersey. The first printing, in 1968, consisted of 5,000 books. Seventeen printings and one revision later, the printing load has grown to 25,000 volumes. "Cookbooks sell by word of mouth," says Tobin, accounting for the popularity of "Forum Feasts."

The Cookbook Store began as a shop in Faneuil Hall, Boston's restored marketplace cum trendy shopping area; branches followed in Providence and Philadelphia (now closing). "Forum Feasts," reports Tobin, is the best seller in all the stores, though Washington readers request more southern cookbooks than readers in other areas.

The popularity of particular cookbooks may depend on the city, but could also reflect the neighborhood. Somehow it seems appropriate that a Georgetown bookstore would carry a $75 tome on caviar, complete with color pictures and everything you wanted to know about caviar but couldn't afford to ask. Or a professional menu planner and food service management book for $110. Cooks more frugal but no less ambitious may lean toward the Culinary Institute of America's "Professional Chef," for $32.50.

Katherine Barrett, buyer for Kramer Books, says that the "new stuff" is selling this season, most notably James Beard. After that, she names the "Joy of Cooking," the first "New York Times Cookbook," and two Pierre Franey "60-Minute Gourmet" books.

And while many people who watch food trends for a living may claim that quick-and-easy or sushi are the hottest things in cooking these days, the book sellers don't concur.

They agree that all cookbooks sell well these days, but the most popular genre seems to be vegetarian, and the hottest items there are both volumes of "The Vegetarian Epicure" and "The Moosewood Cookbook." "Those are the ones really doing well now," says Barrett.

What about special diet cookbooks? "If it says 'gourmet,' " says Tobin, "it sells. And the cookbook for ulcer-sufferers is a great seller. I don't know," she muses, "maybe there are a lot of people in this town with ulcers."