For general information on evaluating, selling, and preserving old books, visit Your Old Books. This RBMS brochure is now online and can answer many general questions about how to determine what books are potentially valuable.
You can locate antiquarian booksellers and appraisers through the Antiquarian Booksellers’ Association of American website or under “Book Dealers, Used and Rare” in your local yellow pages. In order to research what a book might be worth, you can search for your book online. Here are links to three recommended sites: