Search through websites such as bookfinder.com and abebook.com to compare your book to their database. This will help you find the current price for your book. The downfall of this system is that ISBNs were not used until the late 1960s, which makes it difficult to find the exact edition of an old book. In addition, Internet services cannot consider factors such as a book's condition when providing an estimated worth.
Take the book to a bookseller or an appraiser. These people are trained to know the value of books based on condition, age and edition. Because every book is different, having an appraiser view a book in person will give you a more accurate estimate of its value. For a list of antiquarian booksellers in your area, visit the Antiquarian Booksellers' Association of America's website (abaa.org).
Check out the various publications and print sources dedicated to the buying and selling of rare, antiquated and old books. One print source is the Bookman's Price Index, which is published twice a year. Each issue contains the price index for 30-35,000 books. The American Book Prices Current (ABPC) is published annually in the U.S. and the United Kingdom in CD form.