So far I've gotten the Underdark book and I'm getting the Campain Setting soon. Not a logical order I know, but I like the Underdark.
Recently, I've been messing with Races of Fearun, also a nice book.
Magic of Fearun is supposed to be good too, haven't read it yet though. It's in my long-term planning.
So, what are your thoughts on FR books?