Book of Mudora
The Book of Mudora is found in the Library directly south of Kakariko Village. In order to get the Book off the shelf, Link will need the Pegasus boots to bonk the bookcase.
The Book allows Link to translate a few texts throughout the game. Opening Desert Palace requires the book to translate the ancient tablet in front of it. This can only be bypassed with the use of major glitches such as Clipping.