No...it's closer towards the front. Sathius is WAAAY in the back!
From the Pool of Unspeakable Holiness:
Go south, and across the room with the collapsing floor. When you emerge in the large room with the statues, turn right and go down the hallway. Go over the pit trap, and then follow the path to a T-intersection. Turn right again.
The Tome of Earth is in a chest behind a hidden wall at the end of that room. On the left side is a long, blood-stained corridor with blocks that come up to crush you. If you run down that corridor and flip the jackal-head switch at the end, you turn off the trap, and open the secret door, releasing a Demon Lord (blue) and opening up the small room where the chest is. The Tome of Earth is in that chest.
If you need extra hints, go here:
http://www.volberding.com/Whints/Mas...es_Pyramid.htm
-Sazerac