Only a small portion of the millions of books in existence is considered "rare" by appraisers and experts. The definition of a rare book has been argued countless times, but the widely agreed-upon definition states that a book obtains a certain level of rarity only once the demand is greater than the supply. So, the greater the demand and lower the supply, the more rare the book, right? Kind of, but this rule isn't foolproof. Other factors to consider in determining the book's value and rarity are its historical relevance as well as its date of origin and condition. Speaking of condition, the book's physical characteristics can also lead to rare status. For instance, if the book has a unique binding or design, or if it utilized a revolutionary ink or printing process, it could be considered more rare and valuable. Signatures, inscriptions and autographs also add to a book's value and rarity. However, authentication is important if a signature is the driving factor behind a book's price. Rare Vs. Scarce
There is also a difference between a rare book and a scarce book. The age of the book doesn't make it rare in and of itself. However, if the book was printed a significantly long time ago, it can be automatically considered rare. Those books dating prior to the year 1501 would fall into this category. As well, so would books printed in England pre-1641, those printed in the U.S. pre-1801 and those printed west of the Mississippi River in the U.S. prior to 1850.
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