There are many resources online to help you in the pursuit of books for your collection, depending on your area of interest, and too many to list here, but start your search at any of the following bookseller association websites.
Stop first at the ABAA (The Antiquarian Bookseller’s Association of America) www.abaa.org, which is an association of professional booksellers founded in 1949 and now boasts a membership of over 450 specialist and generalist dealers. “The Antiquarian Booksellers' Association of America was founded in 1949 to encourage and promote interest in and appreciation of antiquarian books and other printed materials, prints, and manuscripts; to establish and maintain professional and ethical standards; and to foster friendly relations among both the membership and the bookselling community at large.” Here you can locate booksellers who specialize in your area of collecting. An example is Oak Knoll Books (www.oakknoll.com) located in New Castle Delaware, where you can find a great inventory of books about books and publications by Oak Knoll. Or, Quill & Brush (www.qbbooks.com) located in Dickerson Maryland, where you can find fine first editions, and buy the fourth edition of the Ahearn’s Collected Books: The Guide to Identification and Values. Or, William Reese (www.reeseco.com) in New Haven Connecticut for Americana. At the ABAA website you can also find valuable information about book collecting under the heading “About Rare Books.”
Another tremendous resource is ILAB (The International League of Antiquarian Booksellers) www.ilab.org. Here you can find booksellers from around the world, as their motto says “The World’s Best Books, The World’s Best Booksellers.” In addition there is a glossary of book terminology and a listing of national bookseller’s associations.
Also consider the IOBA (Independent Online Booksellers Association) www.ioba.org. Here you can find a wealth of information, especially a handy reference to book collecting terms.
In addition to the Ahearn’s Collected Books: The Guide to Identification and Values, there are many bound works to consider. For book terminology, John Carter’s ABC for Book Collectors, now in its eight edition, is a classic work. For identifying first editions, Bill McBride’s A Pocket Guide to the Identification of First Editions, now in its seventh edition is also a classic. Matthew Budman’s Instant Expert: Collecting Books, is a good overview of book collecting. There may very well be a guide or bibliography for almost any area of book collecting.