''Things that you can find in a normal bookshop you can't find here,'' says Seymour Bress, owner of Unicorn Books in Gaithersburg, Md. ''We don't have a single dictionary, except a dictionary-encyclopedia of weaving. We don't have any Bibles, but we do have a book on biblical embroidery. We don't have any history books, just the history of samplers.''
Unicorn, which carries only books on crafts, is one of an estimated 1,500 American bookstores that deal solely with a specialty - archaeology, art, Asia, autos, and so on.
Such stores crop up not only in the large cities, but also in places like Vermillion, S.D.; Berea, Ky.; Wendell, N.C. And even the most obscure stores are likely to serve mail-order customers throughout the United States and abroad.
Sometimes foreign customers travel to the store. Unicorn Books, for example, has had visitors recently from places as far-flung as Norway, Saudi Arabia, and Papua New Guinea.
Though the book industry doesn't compile comprehensive statistics, the available figures (from R. R. Bowker's directory of retail stores) indicate the specialty area is the second-fastest-growing segment of the retail book field, just after the burgeoning chains like B. Dalton, Waldenbooks, and Crown.
Bowker's American Book Trade Directory, a sort of Yellow Pages listing of retail and wholesale firms in the US and Canada, counted 1,472 US specialty bookstores last year (not including children's, antiquarian, or foreign-language shops), an increase from 255 in 1961. These figures show nearly a sixfold increase in 20 years. During that period, the total number of bookstores of all types a little more than doubled (from 8,538 in 1961 to 19,048 last year).
Why the growth in specialty stores? A proliferation of books published on birds, cooking, chess, and countless other subjects makes it possible for these shops to offer to limited, specialized audiences a steady stream of fresh titles that usually don't find their way into regular bookstores. Publishers Weekly noted in a December 1981 article that ''with more than 600,000 volumes currently in print, bookstores that essentially duplicate each other's stock cannot . . . offer . . . more than a small fraction of what is available.''
Anne McEntire, editor of the American Book Trade Directory, points out that bookstores are often attractive ventures to entrepreneurs, because a smaller initial investment is required than in many other new businesses. She notes also that there are fewer casualties among new specialty bookstores than among new general bookstores.
The fact that specialized books are rolling off the presses, yet can't be found on general store shelves, has launched even hobbyists and mystery buffs into the book business.
In fact, Seymour Bress opened Unicorn Books in part because his wife, Helene, knew how difficult it was to find information on her craft, weaving. Mr. Bress explains that the largest bookstore in his vicinity carries roughly 200 books in its crafts section, yet only a handful are on weaving or related skills. Unicorn Books, on the other hand, stocks 700 to 800 books on spinning, dyeing, and weaving alone - some of them scarce imports. In addition, it stocks 3,000 volumes on woodworking, carving, metalworking, blacksmithing, jewelrymaking, pottery, needlework, and other crafts.
Carol Brener, owner of Murder Ink Ltd., in New York City, explains that this haven for mystery buffs owes its existence to the ingenuity of founder Dilys Winn, who, ''like a lot of mystery people, was tired of being treated as some kind of moron by regular bookstores and of having to buy one 'worthwhile' book with every 10 mysteries so that the clerk would treat her properly.''
The vast majority of the specialty stores depend on mail-order sales for a part or all of their business. Buteo Books, for instance, in Vermillion, S.D., catalogs 1,000 titles on ornithology and does all its business by mail.
Some specialty shops emphasize scarce and out-of-print books, yet do so without crossing over exclusively into the realm of expensive antiquarian volumes, sought after by collectors for their rarity rather than for reading. Kew Books in New York City, for instance, which grew out of the personal collection of owner John Chancellor, offers hard-to-find books on horticulture and natural history, most of which are priced under $25 and contain information of use to gardeners. A few, however, are priced at several thousand dollars.
While most of the stores surveyed by the Monitor are less than 15 years old, a few are much more venerable. In Cambridge, Mass., the Grolier Book Shop, founded in 1927, is the nation's oldest store devoted exclusively to poetry. Wittenborn Art Books in New York City began nearly half a century ago. One of the newest firms to enter the specialty field is the Computer Knowledge Center, which set up shop in New York City a few months ago to market mail-order books for personal computer owners.
The most highly specialized store in the Monitor survey is Art Catalogs in Los Angeles, which offers catalogs from museum exhibitions around the world - exhibitions of 20th-century art, that is. If it's King Tut you're after, you'll have to look elsewhere. In fact, that may be an unclaimed specialty just waiting for an entrepreneur to grab up.