I buy and sell antiquarian books. Many things go into value, but the basic essential value of anything is what a willing seller will pay a willing buyer.
A good place to start is AddALL
http://used.addall.com/ . This is the best one stop site for a grouping of antiquarian booksellers.
Note carefully: this will give you the price that certain booksellers will sell the book for. That is NOT necessarily its value. I figure that I can sell books on eBay for about 1/3 of the price I find on AddALL.