* Number of Copies: Books with very few surviving copies are obviously rarer than those with many.
* Historical Significance: Books with significant historical or cultural importance are often highly valued.
* Condition: Books in pristine condition are more valuable than those that are damaged or incomplete.
* Ownership History: Books that have been owned by famous individuals or institutions often carry a higher value.
With all that said, here are a few contenders for the title of "rarest book in the world":
* The Gutenberg Bible: This is one of the first books printed using movable type, making it historically significant. There are only 48 complete copies known to exist.
* The Book of Kells: This beautifully illuminated manuscript dates back to the 8th or 9th century. Only one copy exists, making it incredibly rare.
* The Bay Psalm Book: The first book printed in the British colonies of North America, only 11 complete copies are known to survive.
* The First Folio of Shakespeare's Plays: Published in 1623, only 230 copies are known to exist, and it is considered the most important book in English literature.
Ultimately, the "rarest" book is a matter of opinion. It's important to remember that rarity is only one factor that contributes to a book's value.