where is it?=)

I'm going to give you directions from the room with the Holy Water fountain.

Go south, then west. Jump over the collapsing bridge, and emerge in a semi-square room with three exits. Go north from this room (ramp slopes downward, then east. There will be a pit facing you. Walk along the edge of the pit (be careful of the sides, as they will collapse). Go south after you reach the end of this hall.

You will be at a t-intersection. Turn right (I think that's west again). In the big room, there will be an exit to the left. Take that. You'll find yourself in a hallway filled with bloodstains and when you attempt to go down it, huge columns come up to crush you. Run down the hallway as fast as you can and flip the jackal-head lever at the end. This 1) disables the column trap and 2) opens a hidden door back in the room you just left, revealing a demon lord and a chest that may be hard to see.

Inside that chest is the Tome of Earth, what you're seeking.

Hope this helps.