For years, the bible for Oz collecting has been Bibliographia Oziana. This book, which is available from the International Wizard of Oz Club, is still the best guide to identifying early editions of the Oz books, and learning important publication points. Copies may be ordered from the club website: http://ozclub.org/Home_Again.html
I'm very excited about a new book to be published next month called The Book Collector's Guide to L. Frank Baum & Oz. This book is written by Paul Bienvenue and Robert Schmidt, and will feature all of Baum's published titles, as well as later Oz titles and related pieces, with variations and rarities all in color images.
This volume has been in the works for some time – in fact I contributed several pictures 6 years ago or so. I'm curious to see what may have been used.
Copies may be ordered online from Paul's shop, March Hare Books: http://www.marchharebooks.com/?page=shop/disp&pid=page_BaumandOzPg