a pile of schist
If it is a separate book, I'm interested to see what it is about. One thing I'm worried that, if Rick is writing this at the moment, and it ISN'T the next HoO, we may have to wait another year for the next book, if there is one. Unless he has some serious multitasking skills and can write 2 300-ish page novels in a year.
Besides, I can't imagine a Greek Mythology book by him that isn't in the Percyverse. (That's a fun word to say