This Earth Book originally was a Five Spirits Law Heavenly Book buried underground in the Four Styles Solitary Path for unknown years, endlessly sucking in the qi of Heaven and Earth. Thus, it developed into the True Spirit Realm, turning from a secret text into a True Spirit.
The Earth Book inherently was good at defense. Books True Spirit, Empress Tu was so strong that even Emperor Liamg would have problem subduing it. Su Xing with help of Gongsun Huang and three Prehistoric Spirit Treasures barely managed to subdue the book.