As someone else who is sticking with 3.0, I know it can be a bit difficult finding books that won't require a bit of tweaking. You have pretty much all you will need right with the 3 core books. Here are a few other books I would suggest that you could find useful. This, of course, depends on the style of game you are interested in playing.
RPG Specific Books
Monster Manual II - A nice addition of creatures with a different twist to them.
Savage Species - Fun for adding monsters and non-standard creatures as PC races.
Anything from Philip J. Reed - Phil does some fun, creative, and interesting thigs with D&D, and has a wonderful time doing it. Check his stuff out at
www.philipjreed.com
General Sourcebooks
Bullfinch's Mythology - a good source of story ideas and creatures for European styled games.
Any respectable book on the mythology of your choice.
The Encyclopedia of Crystal, Gem, and Metal Magic by Scott Cunningham and
Cunningham's Encyclopedia of Magical Herbs by Scott Cunningham - These are great sourcebooks for creating your own magic items and potions. I found them delightfully useful.
And finally, any fiction book that covers the style of gaming you desire, especialy classic fiction. These help give you an idea of the flavor of a game, plus it's a good excuse to read classics of literature. I know that when I was going to run my swashbuckling game I read just about everything by Dumas.
I hope those help!