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.

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.