The Ancient Skybook is a key story item that houses the knowledge needed to find the path to the sky left behind by the Oocca.
Initially this item is given by Impaz in the Hidden Village in attempt to help Link restore the power of the Dominion Rod which is the key to finding the sky cannon which can be used to reach the City in the Sky after being repaired by Fyer.
The skybook however is missing some of its characters needed to help decipher the text left in that tells of the left behind cannon buried under Kakariko Village.
When Shad is presented the skybook he helps the hero restore the Dominion Rod's power which then can be used to track down the remaining characters scattered around Hyrule and once this is complete Shad can remove the seal of the statue blocking the path to the ruined sky cannon.
In the randomizer a few quality of life changes help speed up the process of accessing the City in the Sky including: Having shown Shad the skybook which leaves behind the powered up Dominion Rod and the skycannon is considered already warped once the skybook is filled up and also gets repaired automatically for free.