I must inquire: is this setting guide supposed to be pre-adventure-path, or post-adventure-path? Because the post-adventure-path setting is completely unrecognizable from the pre-adventure-path state of the world.
The events of the adventure path completely shake up the world into a vastly reformed state: politically, culturally, religiously, metaphysically, cosmologically. Effectively, any setting guide would be heavily invalidated by the adventure path itself.
The Zeitgeist world is very different from, say, Paizo's Golarion. Over in Golarion, each adventure path is much smaller in scope and usually changes the status quo for only a single nation or region. But Zeitgeist's world was was tailor-made for its adventure path, which completely wrecks the world's status quo and replaces it with something else entirely. That would never, ever fly in, say, Golarion. And that is one of Zeitgeist's strengths: allowing the PCs to make vast, sweeping changes on the grandest of scales.
What is the purpose of the setting book, then? The best niche I can think of for a setting book is supplementary material for people running the adventure path, so that GMs are better equipped to run side stories and expand on various plotlines that the adventure path is terribly lacking in detail on (and the adventure path does genuinely miss out on a vast swath of material). Effectively, such a setting book would have to be divided between player-friendly material, useful for coming up with backstories, and GM-only material, for adding further detail to the adventure path.