I'm slowly going through the book and I have to say it's very very good. However, I did notice something that I think is odd. There might be a perfectly good reason for it and I'm just not thinking of it, but it makes me curious.
The Empyrean Opus has a list of spells it contains. Some of these spells, guardian deva for example, aren't on the spell list of anyone who would prepare a spell from a spellbook. I mean only a wizard or magus uses a spellbook. Why would a spell not on a wizard/magus spell list ever be in a spellbook?
Is the only reason it's there so someone who prepares the spell from the book can get the preparation bonus? I guess that makes sense, it just seems odd to me.