 When Frodo is leafing through Bilbos Book in Rivendell, a page with  dwarven runes is shown. The runes translate thus: "Stand by the grey  stone when the thrush knocks, and the setting sun with the last  light of Durins Day will shine upon the keyhole." This is a  reference (actually a direct copy) to a map in the book "The Hobbit"  and the runes tell of the secret entrance into The Lonely Mountain.