I, personally, consider the following books as must-haves:
Player's Handbook 3.5
Dungeon Master's Guide 3.5
Monster Manual 3.5
Forgotten Realms Campaign Setting
Player's Guide to Faerûn
Of course, if you do not want to play in the Forgotten Realms, the latter two should be replaced by the campaign setting of choice (i.e. Eberron Campaign Setting or any of those awesome 3rd party settings (Kingdoms of Kalamar, Midnight, Scarred Lands, etc)).
Beyond that, well, you don't really need anything else.
The Complete series (Complete Warrior, Complete Divine, Complete Arcane, etc) is a good rules addition.
For the Forgotten Realms, the sourcebooks (Silver Marches, Unapproachable East, Underdark, etc, but also Magic of Faerûn, Races of Faerûn, etc) are very good.
And then there are those thousands (or so) of other great (or not so) books out there...
Bye
Thanee