Massive spoilers ahead:
The temple o' death is where you encountered the four sentinels.
In order to enter the death temple, you'll need to place one of your pyramids in the spot where several dead bodies lie, then pull the lever. Then use the other pyramid and you're in.
In here you'll need a rather high perception. Cassandra's grave is in here somewhere, behind a wall.
Dig her up and set her on fire.
To get the spellbook: Remember when you fled the mines through the mirrors? One mirror send you off to the surface. You'll need to return to the surface mirror, which can bring you back to the mines. In there you'll need to find the one mirror that gives you a destination that other mirrors don't. (I forgot what the exact destination is, but it's not 'crypt', nor 'entrance', nor 'library'.
Someone correct me where needed please.
