Ancient Sky Book

In Twilight Princess, the Ancient Sky Book is a book written in the sky language and the key to restore power to the Dominion Rod and reach the City in the Sky. To achieve this, Link must find all 6 missing Sky Characters hidden beneath the Owl Statues scattered across Hyrule. Link obtains this book from Impaz, the old woman who lives in the Hidden Village.

Trivia
The symbol on the cover of the Ancient Sky Book is similar to the symbol that is on Farore's Pearl from The Wind Waker and also is the symbol for the Tune of Echoes In the Oracle of Ages