After a long conversation with the sly old dragon, Yusuf Zuck captured a lot of useful and significant information.
First, the Book of Earth is just a part of The Book of Five Elements, left behind by someone named Blake Kain, who came from another dimension and represented a certain force.
In this world, there are also the Book of Kui Water, the Book of Geng Metal, the Book of Life, and the Book of Flame Fire—four divine books, the whereabouts of which are unknown. Perhaps they are in The Five Continents or perhaps in some unknown continent, unknown dimension, etc.
Second, regarding Chad Lopez and the first Pavilion Master of the Saint Dragon Pavilion, these two individuals were actually Twelfth Level Earth Immortals. They had existed in the Middle Ages and suddenly appeared over five thousand years ago, then they established the Saint Dragon Empire.
After that, the two of them vanished, likely having ascended to another dimension.