* Historical significance: A book with immense historical importance, like the Gutenberg Bible, is priceless to some.
* Rarity: Extremely rare books, especially those with unique features, can be incredibly valuable.
* Condition: A book's condition plays a huge role. A pristine, first edition is worth far more than a damaged copy.
* Market demand: What collectors are willing to pay at any given time influences the price.
That said, here are some books that consistently top lists of the most valuable:
* The Gutenberg Bible: Printed around 1455, this is considered the first major book printed with movable type. Only a handful of complete copies exist, making it incredibly valuable.
* The Bay Psalm Book: Printed in 1640, this is the first book published in British North America. A pristine copy sold for over $14 million in 2013.
* The First Folio of Shakespeare's Plays: Published in 1623, this collection is extremely valuable due to its rarity and historical significance.
* The Book of Kells: An illuminated manuscript created in the 9th century, it is considered a masterpiece of Irish art. While not sold, its value is immeasurable.
Ultimately, "most valuable" can be interpreted in many ways. These are just a few examples of books that have been highly sought-after and incredibly valuable.