Books are technically rare when the demand for the book exceeds the supply. This can happen for several reasons. A book may not have been printed in a very large run, or may be a limited edition. Sometimes, a first edition may have printing errors and the run may have been shortened, making the surviving copies quite rare.
An author's inscription or annotations in the margins by a famous critic can greatly increase the value of a book. Even the bookplate of a famous collector can make the book more desirable.
While age itself is not necessarily a factor in rarity, the date of printing for very early books does impact this. Any book printed before 1501 is considered rare. Any American imprint before 1801 is also considered rare, as are any Confederate imprints.
Of course, it goes without saying that it is more valuable to have a book close to its original condition. In the newer book categories, a signed, modern first edition is often quite collectible. Here in Special Collections, we collect a wide array of rare books that have notable value to SCAD programs.