I would qualify these books as must-haves for 3E gamers and DMs:
1) Midnight- there is nothing else even remotely like it out there- it beats the snot out of all other campaign worlds! Even if you don't play it as is- there are tons of great ideas in there.
2) Book of the Righteous- THE best cosomology out there for any game.
3) Legions of Hell and Armies of the Abyss- go along with BotR, and full of nasty new fiends.
4) Monster's Handbook from FFG- great resource for DMs wanting to put new twists on monsters, and lots more useful than Savage Species.
5) Spells and Spellcraft- lots of interesting ways to twist and tweak magic, including new spells, rules for familiars, item creation, etc.
6) Any of the Path of books (Sword, Faith, Magic, Shadow). They have great ideas about how to give the various classes a more personalized touch, lots of new PrCs, a few new core classes, and feats.
7) Book of Vile Darkness- makes DMs grin evilly just looking through it!
8) Ultimate Equipment Guide from Mongoose- a great resource for all sorts of non-magical equipment, but that would still be incredibly useful to adventurers. Much better than the Arms & Equipment Guide- kinda like the old Aurora's Catalog.