Where is the fourth book for the zenmaster quest?

It's in Cet's pyramid, behind a hidden wall. The wall is opened when you go down a long hallway that's bloodstained, with columns that rise up to crush your party. Run down that hallway, flip the Jackal head at the end, then return down it. A Demon Lord will be there, along with the newly opened chamber. A chest in that chamber has the Tome of Earth.

For more specific info, check out Rick's site at http://www.volberding.com

-Sazerac