Know the age. The age of a book alone rarely determines value. However, according to the Oklahoma State University Library, volumes published in the United States before 1850, or published elsewhere before 1800, should be given careful review. Demand is a big factor in the value of your book. If the edition and title is of interest to collectors, the value would be improved.
Consult printed resources. Most people researching a book go immediately to online resources. But there are two other very important references that should not be overlooked when pricing antiquarian books. American Book Prices Current is an excellent resource and provides actual prices from books and other printed materials sold at auction in the U.S., U.K. and Europe. Another good reference is Bookman's Price Index, which is published three times a year and provides prices and availability of antique books in the U.S., U.K. and Canada. The reference gives the actual retail price for books and their condition. Both of these references can be found at most public libraries.
Check online sources. The Internet is filled with excellent sources to help you establish the value of books. AddALL is a good database that returns prices on books from more than 40 online booksellers, while AbeBooks offers prices from more than 10,000 booksellers (see links in Resources). One of the best known is Alibris.com, which provides music, book and movie searches and also has links to book collecting guides.
Consider an appraisal. For special books and collections it may be appropriate to hire a trained book appraiser to determine the value of your treasures. This may be necessary for insurance purposes or if you're considering a donation of books to a library or museum.