Hi Peridot!
As of right now, the text in the book is the main source of information about the world. I find that the setting is largely mutable after the 3 important inclusions (Travelers, Monsters, Shrines). Feel free to adapt anything at your table with these elements in mind!
My check list looks something like:
- What sort of civilization fell here
- What caused the fall and why is it still hanging around?
- What sorta fucked up stuff happened as a result?
- Who is left after the fallout and what dangers do they face?
The official first adventure™ will have more setting info and lore to run specific adventures in Amarna (one of the countries mentioned in the core book). It is still underway!:)