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 six missing Sky Characters hidden beneath the Owl Statues scattered across the Hyrule overworld. Link obtains this book from Impaz, the old woman who lives in the Hidden Village, after showing her the reactivated Dominion Rod.

Sky Characters
Sky Characters, also known as Sky Writing, form the basis of the ancient Sky language, which is the native language of the Oocca. Shad is likely the only Hylian still able to read this language, which is written in the Ancient Sky Book; indeed, he is the only one who can help Link with the matter.

After Shad restores power to the Dominion Rod, Link must then find six Owl Statues scattered across Hyrule and add the Sky Characters hidden beneath them to the Ancient Sky Book, so that he can find the Sky Cannon and gain access to the City in the Sky.

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