Modules are the easiest books to adapt to 3rd edition, and there are a few that are keepers, like the Return to the Tomb of Horrors. The monster books are good to keep because they can give you ideas for monsters, though the old stat blocks are close to worthless, except in a general, giving you ideas sort of way.
The campaign setting stuff is also worth keeping if you liked the setting. The stats are close to worthless, but most of those books were more flavor text than stats anyway. The Forgotten Realms has obviously been relaunched in 3E, though the amount of 3E material doesn't even come close to rinalling the amount that's out there for 2E, so there's a lot of information that would just be lost if you got rid of it.