Gather information about your book. This should include the author and title, publisher, place, and date of publication, size, type of binding and the condition of the book. The condition of the book will be subjective but do your best to accurately describe its physical appearance.
Consult a reference book to get a general idea of your book's value. Some titles you may consider for this are "American Book Prices," "Used Book Price Guide" or "Book Prices: Used and Rare."
Find a reputable antiquarian bookseller. This is the best place to have your book accurately appraised by a professional. He can do a first-hand evaluation of your book, relieving you of the guesswork about its condition. The Antiquarian Bookseller's Association of America (ABAA) offers a list of booksellers across the country, along with other information pertaining to rare books.
Submit your book's information to an online book appraiser. If there is no professional appraiser in your area, there are a number of websites that offer this service. This is where an accurate description of your book will be most important to finding its true value. Be sure to only use a website that appears reputable. Check to see if the website asks for the information mentioned in Step 1, has a physical address as well as an online one, and if it offers any other book-related services like auctions or collector advice.