Well, I just started DMing a Forgotten Realms campaign, so I use whatever FR books I can my hands on, anything from 1st Edition, 2nd Edition, and Dragon/Dungeon/Polyhedron articles.
I think books that deal with the flavor/setting of the game can always be used, regardless of what edition it is in.
In regards to FR, there is ALOT of history and events that the 3rd Edition books either do not touch upon or just "gloss over". Not that that is a bad thing, because the amount of info on FR, in my opinion, is very daunting. Having minimal information is sometimes easier to work with than having dozens of books to peruse through.