What bookdealers really mean. A dictionary by Tom Congalton and Dan Gregory (Between the Covers), with additions from other sources
One of two book collecting guides written by Allen and Patricia Ahearn. The most recent editions are Book Collecting 2000 and Collected Books: The Guide to Values. The first is an excellent one-volume overview of book collecting, with price estimates for the first books by thousands of authors. The second is a straightforward price guide, with the added bonus older price data. Every major book dealer has these guides in their personal reference collection and usually right at his or her desk (yes, even in the age of the Internet), which ought to tell you something about their worth.