To find out the fair market value of old books, take the books to a professional appraiser. You can also monitor the prices of similar books through online auctions and book dealers. Biblio.com offers a book value tool to find book prices, as of July 2015.
Visit the Biblio.com homepage, and enter any information you have on the book, such as the ISBN or the author, title and keywords. Check the hardcover, first edition or signed boxes to filter the search. Results typically include a short description of each book, as well as the seller and price.
Look for books that are in similar condition to your books, and gather a range of prices. Consider any values you obtain by monitoring book prices to be a rough estimate of your book's value. The book may not sell at the listed price you found, however, and this price can rise or fall depending on the book's popularity.
The book's condition is the most important factor in determining its value. Collectors typically want books in good condition, so an otherwise valuable book may be worthless if damaged. The condition of the dust jacket also has a significant effect on the book's value.