Sorry, went to bed. Everything is legal, just don't don't don't break the game, okay? So all the official player books, splatbooks, Dragon magazines, etc are fine. Go easy on the Dark Sun/Eberron/Forgotten Realms themed stuff, though. I want to reiterate that this is a Points of Light setting, so while I don't mind you guys taking classes, themes, and backgrounds from those books and using their mechanical benefits, you'll have to fluff them accordingly to be part of the Nentir Vale setting.