Pittsylvania County Library, like most public libraries, does not have the expertise on staff to judge the value of old books. However, the websites listed below can set the determined seeker on a path which should, in the end, provide a close approximation of current value.
Bills itself as "the world's largest online marketplace for used, rare, and out-of-print books."
Offers "over 30 million used, new and hard-to-find titles."
Claims to have "over 50 million new, used, rare, and out of print books."
Offers a good discussion of many of the sites listed here, orienting the new user to web and non-web resources for rare books.
Chegg both sells and rents textbooks.
One of the most famous independent booksellers in the US provides a rich source of information and values for rare materials.
The TomFolio Website is a cooperative of international professional dealers in used, out-of-print, and rare books, periodicals, and ephemera.