THE NOSTALGIA BUSINESS is booming in paperback publishing as never before - aided, no doubt, by expired copyrights but also reflecting a yearning towards the past among people who buy books. The nostalgia craze extends into a bewildering variety of fields, but almost all of it reflects the axiom that today's junk may be tomorrow's treasure.

The leading publisher in this field is Dover Books, which emphasizes the out-of copyright in its voluminous catalogue, but other publishers are rapidly following suit. Subject matter ranges from the old postcards and baseball cards to pages to 40-year old movie fan magazines, the pinup pictures painted on World War-II aircraft and the dolls that children played with 150 years ago. A few sample titles:

Classy Chassy, by Ian Logan and Henry Nield (A & W Visual Library, $5.95). A very specialized form of pop art and one with an unusually brief life-span was the "girlie" art which adorned American fighting planes during World War II - painted there by the men who flew or maintained the planes, and tolerated or encouraged by higher officialdom as a boost to morale and an expression of the indomitable American spirit. As art, these pictures were negligible (many were based on popular pinups), but as memorabilia they are worth a backward glance, and they are well reproduced in this unusual collection of softcore pornography.

Classic Baseball Cards: 98 Collector's Cards Authentically Reproduced in Full Color, Perforated for Easy Detaching, edited by Bert Randolph Sugar (Dover, $2.50). The time-span covered in these remarkable replicas is from 1880 to 1940, and the subjects include such favourites as Babe Ruth, Leo Durocher (when he was an infielder, not a manager), Ty Cobb and Dizzy Dean, as well as old-timers Christy Mathewson, Napoleon Lajoie and many others. None of the sets represented (mainly from cigarette or chewing-gum packages) is complete, and there are some notable players omitted (Ted Williams, for example), but browsing through these garishly colored replicas one can almost sense the indefinable odor of bubble gum associated with the originals.

Hollywood and the Great Fan Magazines, edited by Martin Levin (Arbor House, $5.95). A collection of articles photographically reproduced from such magazines as Photoplay, Silver Screen and Modern Screen, these stories ("Kay Francis Outlines Her Conception of an Ideal Honeymoon," "Career comes First with Loretta," "Why Girls Fall in Love with Robert Taylor") indicate that nearly anything can be considered valuable if it has the right vintage and evokes memories in enough people.

Easy-to-Make Dolls with Nineteenth Century Costumes (Dover, $2.75). Instructions and patterns for making and clothing sewn and stuffed dolls based on originals that date from the 1820s to the 1890s.

Cut & Assemble An Early New England Village, by Edmund V Gillon Jr. (Dover, $2.50). These heavy paper models in full color, based on actual buildings from Massachusetts and Connecticut, can be assembled to form a miniature village that is highly typical, perhaps partly because it never existed in quite this form and arrangement.

Thirty-Two Picture Postcards of Old Washington, D. C.: Ready to Mail, edited by Robert Reed (Dover, $2). Horse-drawn trolleys, the old Smithsonian Building standing alone in a field of daisies, the Willard Hotel in 1904, looking much as it does today (but how much longer?) and Georgetown University (Ca. 1900) standing almost alone at the top of its hill are among the nostalgic scenes on these old postcards. Similar collections exist for New York, Chicago, Philadelphia and other subjects.

Depression Modern: The Thirties Style in America, by Martin Greif (Universe, $6.95). From floor lamps, theater seats and ash trays to massive buildings and even steamships, the streamlined structures in this compendium (attractive mainly for its pictures but with a long and excellent introductory essay) constitute, in a sense, America's answer to the Bauhaus. Looking back, it seems not such a dazzling answer as we might once have thought, but one with durable values.

The New York World's Fair, 1939/-1940 in 155 Photographs by Richard Wurts and Others, selection, arrangement and text by Stanley Appelbaum (Dover, $5). A curiously dated but appealing look at what was considered "the world of tommorow" yesterday.