When we decided to make this month's Pathfinder Campaign Setting volume, Occult Mysteries, the development team had perhaps the most fun I can remember in almost 4 years at Paizo coming up with the different setting-specific mysteries we wanted to explore in the book.
One of the largest challenges was determining what, in a world where magic, gods, divination, and the afterlife were demonstrably real, constituted the "occult." In our world, research into demons and devils is certainly occult, but is it in a world where one of the most powerful nations in the Inner Sea is openly and proudly in cahoots with the legions of Hell?
In the end, we decided that "occult" in Golarion meant anything that wasn't known to the general population. So while your average commoner pig farmer doesn't know how to cast a spell, he's aware that magic exists. He's not, however, likely to know that some magicians can augment their spellcasting by quickly analyzing the mathematical constants that bind reality together, or that if the stars are in the right alignment certain types of magic are more potent than normal.
As much as we wanted to reveal a lot of the hidden truths behind some of Golarion's longest standing mysteries (Why did the gnomes leave the First World? Who created the multiverse? What's the deal with the Aucturn Enigma countdown clocks in Osirion?) we also wanted to maintain the mystery that has inspired players, GMs, and ourselves and our writers to use these mysteries as jumping-off points in their own adventures. Thus, we made sure that every answer we provided left another (sometimes several) in its place.
Subscribers have already started getting their copies of Occult Mysteries, and the discussions about what reveals they're most excited about and what questions these revelations have raised are exactly what we'd hoped for. Make sure you pick up a copy of Occult Mysteries and see what lies hidden just below the surface of the Pathfinder campaign setting.