The Book of Mudora is a collection of Hylian legends and lore that was compiled during the early years of Hyrule's history, before the Triforce was discovered by Ganondorf. In these ancient days, the Hylians wrote in their own language. The Book of Mudora acted as a translation guide between the ancient Hylian script and the common language of Hyrule.
A well-known verse from the Book of Mudora later inspired many to aggressively search for the wish-granting Triforce:
In a Realm beyond sight,
The sky shines gold not blue.
There, the Triforce's might
Makes mortal dreams come true.
According to a sage named Aginah, the Prophecy of the Great Cataclysm says that "the Hero will stand in the desert holding the Book of Mudora." The Book of Mudora contained a message for this legendary Hero of Hyrule, telling him about his quest against Agahnim. This precious book survived through many centuries, eventually finding its way to the library in Kakariko Village.