The Books of Vanitas are a pair of cursed grimoires in Jun Mochizuki's The Case Study of Vanitas, formerly possessed by The Vampire of the Blue Moon. They have the power to rewrite the World Formula, including the alteration of vampires' True Names, but can only be used by The Vampire of the Blue Moon and their kin.
The Books are currently in the possession of Vanitas and Mikhail, the human kin of the deceased Vampire of the Blue Moon.
The Books of Vanitas both have jet black pages and blue leather covers with silver clockwork adornments. A silver chain attaches to both Books at the centers of their spines.1
The human Vanitas's Book has metallic silver covering the top and left portions of its cover. The middle of the cover is open, showing the leather underneath, and the bottom right is covered by a gearlike mechanism. When the Book is closed and latched, the upper part of this mechanism remains flush with the edge of the cover, taking up much of the Book's right side. There is also a crystal embedded on the left edge of the cover, near where the chain connects on the Book's spine.1
According to legend, The Vampire of the Blue Moon created The Books of Vanitas as a tool of revenge against the vampires that threw them from their village, as well as vampires of the crimson moon in general. It is unclear how true this might be.
To be added
The Books of Vanitas have the power to interfere with vampires' True Names.1
It is broadly believed that there is only one Book, simply referred to as The Book of Vanitas.
A mythologized version of The Vampire of the Blue Moon's story has been spread as a bedtime story. In this myth, The Vampire of the Blue Moon supposedly stated that he who holds the Book of Vanitas will one day become the one to destroy all vampires.1 Aside from the presence of only one Book, it is unclear what parts of this legend might be true.