Intrinsic Importance: The most essential factor in determining rarity is the book's intrinsic importance, or how important the book is considered to be in its field. Only books with some acknowledged importance will have a consumer demand that creates market value and a sense of rarity.
Age: Surprisingly to many people, the age of a book has very little to do with its value. The other factors are typically more important considerations of rarity. Dealers, collectors and librarians, however, do use some broad time spans to establish dates of likely importance: e.g., all books printed before 1501, English books printed before 1641, books printed in the Americas before 1801 and books printed west of the Mississippi before 1850. These dates are rough guidelines at best and are always subject to the overriding factors of intrinsic importance, condition, and demand.
Scarcity: Scarcity does not equal rarity. A book known to exist in only a few copies may have value if it has importance and is in demand. A book without importance or demand has little value regardless of how few copies survive.
Condition: Condition is a major factor in determining a book's value along with intrinsic importance, supply and demand. Condition refers to both the book's external physical appearance and the completeness of its contents. A book in "fine" condition is complete in all respects, has no tears or other signs of misuse or overuse, and is in an original or appropriate and intact binding. A book that has been rebound or is in less than fine condition must be very important or in high demand to be of substantial value.
First Edition: In the strictest sense, "first edition" refers to a copy of a book printed from the first setting of type, constituting the first public appearance of the text in that form. Subsequent changes to the printed text through corrections of the original typesetting produces different "states" and "issues" but not a new edition.
The liberal use of the term "first edition" has made it seem synonymous with "scarce" and "valuable." This is by no means the case. Most books appear in only one edition. Collectors of literary works especially are interested in first editions, and there is a lively and well- documented market for these books. Condition plays an even greater role than usual in determining the monetary value of literary first editions. If an author revises the text for a later edition, it may be of interest too.
Fine Bindings and Illustrations: A book can have physical characteristics that lend importance - a special binding, first use of a new printing process, an innovative design, an autograph or inscription.