Book of Mudora

The Book of Mudora is an item that appears in A Link to the Past.

Location and Uses
The book is a collection of ancient Hylian lore and stories, as well as a guide to translating the ancient Hylian Language that appears throughout the land of Hyrule. It is found in a library south of Kakariko Village, and is used to allow Link to enter the Desert Palace (the second dungeon in the Light World) and also to obtain two of the three Magic Medallions. There is also text in front of the Master Sword's pedestal which Link is able to read using the book.

A Link to the Past comic
In the A Link to the Past comic, Link finds the Book of Mudora after he saves the Librarian from the burning House of Books. Once the fire ceases from the library, the librarian theorizes that Agahnim's soldiers might have been after the Book of Mudora,   and explains that it is the key to the ancient Hylian language which   mentions that the hero will stand in the desert holding the book. Link takes the Book of Mudora and mentions how Sahasrahla had told him the Pendant of Power was in desert ruins, leading the librarian to claim Link as the legendary Hero of Hyrule and calmly leaving the book in the young hero's hands.

When Link reaches the Desert of Mystery, he uses the Book of Mudora to translate the runes written upon the ruins of the Desert Palace. Upon doing so, the ruins begin to sink to reveal a scorpion statue that has the Pendant of Power hanging on it.

Theory
Several books seen in Ocarina of Time 3D appear similar to the Book of Mudora. These can be seen in Impa's House, in the Lakeside Laboratory, and in Dampé's House. These books show the Sheikah Symbol on one side while the other side cannot be seen.

Trivia

 * Mudora (also known as Medora) comes from the ancient Greek language, which means "holder of knowledge", "tutor" or "holder of high wisdom", and the Book of Mudora indeed is "a holder of knowledge" that teaches Link the ability to translate the ancient Hylian language.
 * In official artwork, one side of the book carries a number of unidentified symbols.
 * A book with a similar appearance exists in Lon Lon Ranch in Ocarina of Time 3D. It's the second in a stack of books on a table inside the house.  It can only be viewed in the game resources or by having the camera clip through the top book.  The top one is also similar, but purple.