Several factors can help determine if a book is classified as "rare." This could be scarcity (only a few exist), autographs or annotations written in the author's hand, a special printing or binding process, subject, distinctive illustrations, format, or a unique provenance (origination and ownership).
Professional appraisers can determine the value of rare books. The Antiquarian Booksellers Association of America and the Appraisers Association of America include information on finding an appraiser. Library policy prohibits librarians from providing appraisals for patrons, donors, or the public.
Your Old Books A guide produced by the Rare Books and Manuscripts Division of the American Library Association.
ABC for Book Collectors John Carter’s seminal glossary of book and book-related terms.
AbeBooks.com Online marketplace for new, used, and rare books.
Book Terms Illustrated A visual glossary from Powell's Books.
IOBA Book Condition Definitions From the Independent Online Booksellers Association.
IOBA Book Terminology Common terms in the rare book trade.
Preservation Leaflets The Northeast Document Conservation Center provides advice on the storage, handling, and preservation of rare and archival material.
Rare Book Condition Standards "Fine," "good," "book club" edition – what do these terms mean? Powell's Books has compiled a glossary of terms used in the description of rare books.