Very few of the books published since the invention of the printing press would be considered "rare," according to the late Peter VanWingin, former curator of the Rosenwald Collection in the rare book and special collections division of the Library of Congress and author of a pamphlet titled "Your Old Books." The appraisal value of rare books and magazines depends on several factors, including condition, edition, scarcity and demand. Not every old book is rare, and not every rare book is valuable. With a bit of research, or a professional appraisal, you can learn the market rate for your book.
Determine the publication year and print edition of a book by looking at the copyright page. It is located on or after the title page, though it is sometimes placed several pages after that. Check if the book is marked as a book club or special edition publication. Write this information down and include the title, author, publisher and location.
Use the information you gathered to search an online database, such as the International League of Antiquarian Booksellers, for your book or books by entering the title, author and publication year. Scan the results for the closest possible match with your book---book values are determined by either the item's documented auction price or current listing by an online distributor. Choose three or four entries that are the closest to your book in condition, year and edition, and print them out for your records.
Visit an academic or public library and look for a guide to book values. A few reliable guides are: "Mandeville's Used Book Price Guide," edited by Richard Collins; "The Insider's Guide to Old Books, Magazines, Newspapers and Trade Catalogs" by Ron Barlow and Ray Reynolds; and "Collected Books: The Guide to Values" by Allen Ahearn and Patricia Ahearn. See if your book or books are listed in these resources or others like them. Refer to the condition guide for suggested price evaluation in relation to your book.
Consult a professional appraiser. Market values can shift over time before book value guides can be updated and reprinted. Get a second opinion if you find evidence that your book is valuable---it might be worth more than you thought. Insurers require a professional appraisal for a policy.
Be wary of online antique appraisal websites, as many of them are not legitimate resources.