American Book Prices Current. New York: Bancroft-Parkman. (annual)
Includes foreign titles.
L'Argus du livre de collection. St-Romain-au-Mt d'Or, France: Artprice. (annual)
Covers auctions mostly taking place in France
Book Auction Record: A Priced and Annotated Annual Record of International Book Auctions. Folkestone, Eng.: Dawson, 1903 – 1999.
International book auctions up to the end of the 1997 season
Bookfinder
http://www.bookfinder.com
Used and rare book search service with over 70 million titles.
Bookman's Price Index: A Guide to the Values of Rare and Other Out of Print Books. Detroit: Thomson Gale. (annual)
Prices and availability of books in the U.S., Canada, and the British Isles.
Jahrbuch der Auktionspreise für Bücher, Handschriften und Autographen. Stuttgart: E. Hauswedell. (annual)
Auctions in Germany, the Netherlands, Austria, and Switzerland.
Your Old Books
http://www.rbms.info/yob.shtml
Rare Books and Manuscripts Section (RBMS) of the Association of College and Research Libraries has provided a guide that addresses some frequently asked questions about rare and older books and their values.