Your Old Books This RBMS guide addresses some frequently asked questions about rare and older books and their values. The answers are meant only as general responses to these questions, and many possible exceptions are not described.
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Both of these book sources give the chart for Harper & Brothers letter code system and Zempel has a pretty detailed entry on Doubleday.
First Editions: a guide to Indentification by Edward N. Zempel and Linda A. Verkler.
A Pocket Guide to the Indentification of First Editions by Bill McBride.
Modern Line Numbering An informative site form the Beinecke Library catalogg manual that explains many of the publisher's codes.