Nothing is more common in this city of transients than the sight of cardboard boxes filled with books the owner is saving to read "some day."

Add to the 65 cartons of books brought with you on the last move, the titles picked up at bookstores, newsstands, garage sales and remainder tables and even a diehard pack rat may recognize the need to whittle the home library down to manageable proportions.

The question: How can we know if among the books we finally, painfully reject there are any that might be worth something?

First, dealers make a distinction between "used" books--secondhand books to be sold again as reading copies, wonderful buys if you think of books primarily as something to read--and "rare" or scarce books, out-of-print volumes of interest primarily to collectors, usually at collector prices. Rare-book dealers are often listed as "antiquarian" dealers, but rare is not necessarily old--nor necessarily valuable. In terms of cash value, rare means both hard-to-find and desirable to collectors . . . in short, collectible.

General areas in which collectors show special interest: books with fine leather bindings, incunabula (books printed from movable type in the first 50 years of printing), illustrated books (including old children's books with color or line drawings), military history, early science books, rare books of photography and first editions, particularly of prominent modern novelists (and particularly their first books, usually issued in small printings).

Undesirable to most dealers: old book club editions, Readers' Digest Condensed Books, school texts and technical books, which are constantly revised and out-of-date. Many store owners turn up their noses at Bibles, encyclopedias, how-to books (except very old ones, which preserve lost technologies) and recent bestsellers.

Even titles that have brought high prices elsewhere may not be worth much if they are in poor condition, one of the most important factors in determining a book's value. Collectors classify condition as mint, fine, very good, good and fair.

Essentially: You will get much less for a book if pages are loose, missing or folded, if the binding is loose or detached, if there are tears, stains, mildew or writing in the book. (A name in the front detracts, but not as much as underlining and notes in the text--though these may have value in themselves if someone famous made them.) Autographs or inscriptions by famous authors increase value. With fiction, first printings of first editions are the ones that count; this is not so important a factor with nonfiction.

The presence of a book jacket in good condition affects the price considerably, which must come as terrible news to generations of bookworms who have routinely removed and discarded book wrappers.

"After 10 years, the jacket is probably worth as much as the book," says Allen Ahearn, whose Bethesda shop, Quill & Brush, specializes in modern first editions. "After 10 or 15 years, the jacket is sometimes worth four or five times as much as the book. A Fitzgerald novel from the '20s and '30s without a jacket might sell for anywhere from $50 to $100; with a jacket, from $400 to $1,500. That's because you don't find a Fitzgerald dust jacket any more, so when they show up there's a tremendous premium on them."

Don't assume that only "major" authors like Hemingway and Faulkner are collectible. I learned that my 1961 copy of Larry McMurtry's "Horseman, Pass By" (later filmed as "Hud") was worth at least $100, and would have been worth at least $50 more if there weren't a slight worn spot on the front cover and a slight warping of the binding (possibly from dampness).

Jim Gscheidle, of Imagination Books in Silver Spring (a wonderful store for reading copies), says "Another Roadside Attraction" by Tom Robbins is a good example of how books can increase in value. When "Roadside" was first issued in 1971, in a small printing, it sold very poorly and returns to the publisher were probably lost or destroyed. When Robbins' second novel, "Even Cowgirls Get the Blues," was well reviewed, heavily promoted and successful, people became curious about Robbins' earlier work. A new paperback of "Roadside" was issued, but as a Robbins cult developed, some customers decided they wanted hardcover copies and started checking out used bookstores.

A dealer with one copy of the book and five customers might learn that another dealer was offering it in his catalog for $30, so he mightcharge $10 more because of demand. If he got his price, the new "realized" value of the book would be $40, the price other dealers who knew about the transaction would quote. For about six months, copies were selling for $300; the price has now dropped to $200.

If you suspect that any of your books have particular value, you'll probably get a better price by selling them to a specialty dealer. Dealers who specialize usually won't buy a general collection of books, but are more likely to pay top dollar for individual copies. They know what their market can bear and often send catalogs to customers interested in that specialty.

Says Quill & Brush's Ahearn, "If there is a book I can move, I will probably try to sell it for $25, and pay half that, whereas a used bookstore might pay only $1 and try to sell it for $3 or $4."

Other dealers tend to be vague about quoting a price for individual books, preferring to make quotations for whole collections.

No matter where you go, it's to your advantage to know something about the value of your books. Although some book dealers will give you a fair price for a book they can make a good profit on, others may try to offset the high costs of an iffy business by picking up "finds" cheap. Judgments about any single title may vary radically, depending on a dealer's level of expertise, cash-flow situation, inventory and clientele. Even within a store, prices may vary considerably, depending on who you're talking to.

Basically, the store with higher prices can afford to pay you more for your books, but also is more likely to reject them. Paperbacks? You'll get about 10-25 cents each for the drugstore variety. You can always donate your books to a school, public library or one of the many book sales. For Vassar's Book Fair (April 22-29), you can get a pick-up by calling 223-6274.The IRS says you can deduct the fair-market value of donated books.

But don't consider books the only items of value in those boxes you're carting around.

Says Mike Keck, whose Alexandria store, From Out of the Past, buys and sells old magazines and other paper memorabilia:

"Many estates throw out all the good stuff (obscure catalogs for odd products, postcards, old letters, and turn-of-the-century corporate letterheads) and hang on to relatively worthless items like old encyclopedias. But inheriting paper ephemera is like owning mineral rights to coal. The more effort you put into processing the material, the more return you'll get from it."

The same could be said for your old books. Meanwhile, hang on to your dust jackets.