Some price quides are compiled from auction results. Again you must be careful because the guide may include books which were not sold because the bids did not exceed the reserve price. There are both general stock and specialty price guides. Published annually since the 1890's. Compilation of book auctions.
Most books are high price items.
Compilation of dealer's catalogs. Bookman's Price Index Daniel McGrath. Compilation of dealer's catalogs. Covers medium to high price books. Collector Books, PO Box 3009, Paducah, KY, 42002-3009 Published annually, 7th edition copyrighted in 1995.