To get a rough idea of a book's value, here are three reference books:

* Van Allen Bradley's "Handbook of Values" (a revised edition has just been published)--Estimates value of books in fine to mint condition.

* "Used Book Price Guide" (3 volumes, by Mildred S. Mandeville)--Estimates value of books retailing for $25 and under.

* "The Book of First Books," by Allen Ahearn ($6.50 at Quill & Brush, currently being revised)--Gives general information on first editions (and how to identify them) and estimates values for the first books of 2,000 American and British authors.

For more thorough researching: "American Book Prices Current," an annual listing of the (sometimes misleading) top auction prices paid for expensive, mainly very old books; "Bookman's Price Index," annual compilation of dealers' catalogues, listing prices they ask, not what they got; "Bookman's Weekly" and "AB Yearbook," two Antiquarian Bookman publications in which virtually all dealers advertise; "Directory of Specialized American Book Dealers"; Overstreet's "Comic Price Guide" and "Paperback Price Guide."

The Library of Congress used-book section and some other libraries have many of these references. Some dealers will let you browse through their copies.

Besides being listed in the Yellow Pages, rare and other used-book and magazine dealers are listed on a flyer available free (with stamped, self-addressed envelope): From Out of The Past, 6440 Richmond Hwy., Alexandria, Va. 22306.

For a free copy of the membership directory of the Antiquarian Booksellers' Association of America, send a stamped No. 10 envelope (with 54 cents postage) to: ABAA, 50 Rockefeller Plaza, New York, N.Y. 10020.