One of the easiest ways to estimate the market value of your book is to determine the offering price of other copies of the book in similar condition. Several online book-selling services can be searched for this information:

AddALL 
Abebooks.com 
alibris
Bookfinder.com

If you are unable to locate another copy of your book for sale, your next step might be to contact a dealer that specializes in books of a similar type:

Antiquarian Booksellers’ Association of America 
International League of Antiquarian Booksellers 
International Book Collectors Association List of A+ Book Dealers