The Book of Leaves is an enigmatic and mysterious tome, that Leonardo and many key characters thinks might change the known history albeit in different perspectives and for which he and his friends set on a challenging journey and face many dangers. The Book is supposedly hidden in the Vault of Heaven on a foreign continent unknown to the Old World - the New World (America continent).
The Book's origin is speculated by many parties in the series. Ar-Rahim, a prominent member of the Sons of Mithras provided an esoteric origin of the the Book where it is the collection of cognitive memory of ancient knowledge passed down from one generation to another that aim to revitalize again mankind after the Fall. On the other hand, Pope Sixtus believed that the Book may have Enochian origins as he proposed that it was written by the Nephilim, the offspring of angels and men. But Count Riario offered another theory about the Book of Leaves as he believed it was transcribed by the elders of an ancient civilization in Crete, nine thousand years before Christ, in a place that later became the lost city of Atlantis.
A page of the Book made it first appearance in the bowels of Castel Sant’Angelo, where the pope keeps his Secret Archive. The page is written in an alien language that Leonardo has never seen, but with the wave of the pope’s hand, the writing magically switches to Hebrew, then to astrological symbols, and even to strange hexagonal patterns (identified to be the Chinese Bagua).