To compare what used-book dealers would pay for a fairly desirable batch of books, free-lance writer Pat McNees took 19 hardcover books to 11 used book shops in the District, Maryland and Virginia. Ten of the books were of very little value in anyone's eyes. The other nine included several first editions of modern fiction, of value to some dealers and of little or no interest to others.

Some dealers offered as little as $6-$15 for the lot. A sampling of higher appraisals:

* $20 for "Skull-Face and Others" by Robert E. Howard, in an unjacketed fair copy of a first-edition, science-fiction classic for which the top auction price paid (for a book in much better condition) was over $300. Plus $10 for the rest of the books, from a store that sells both used and rare books.

* $30 for the lot from a dealer who said she would offer more if further research indicated some books had more value.

* $40 for "Skull-Face" and 50 cents for Archie Carr's "So Excellent a Fishe" from a Wheaton science-fiction dealer who said he would offer more for the Howard book if he didn't already have another, better copy.

* $55 for first editions of Mario Puzo's "The Godfather," John Cheever's "Wapshot Scandal" and Alexander Solzhenitsyn's "One Day In the Life of Ivan Denisovich," a limited edition of Joyce Cary's "The Horse's Mouth," and the Howard book. From the wife of a dealer who specializes in first editions and was interested primarily in the Puzo and the Howard.

* $144 for the lot, including $50 for the Howard alone, from a dealer who spent a lot of time looking up first editions in his reference books.

"This was a very informal survey," says McNees. "And industry gossip being what it is in the used-book trade, I'm not sure some dealers weren't tipped off that a price comparison was in the works. Still, the range of offers--and the different selection of books for which offers were made--was varied enough to make it obvious that you should take a little time to find your buyer if you think your books might be valuable."

A couple dealers, she says, "gave the impression that the only way they could maintain profits was to take advantage of the naivete' of customers who come in eager to unload books."

Several dealers told McNees they couldn't offer what the books were worth and suggested dealers who might, such as Second Story Books, a chain of used-book stores that maintains a big inventory. (It sells both inexpensive and collector editions, usually pays one-fourth cover price for review copies of popular current titles and charges customers half-price, unless the book has been remaindered.)

"One dealer was visibly annoyed," says McNees, "when she realized I was comparison-selling without telling her so in advance. On the other hand, I was told that if some dealers knew I was comparison-selling, they wouldn't give me the time of day."