I'll go with what Grimstaff said.
Tome of Horrors, for sure. All around excellent monster book. The hard boud copy is a pleasure to flip through, but for 3.5, you should ge the PDF to back it up. The hardback copy was never updated from 3.0.
Wilderlands Box Set. More setting in one box than you'll probably ever use. 9 Double sided posters gives you 18 usable maps, and 2 nice thick softbacks to tell you what's on the maps. Don't forget The City State of the Invincible Overlord book for a big, nasty, gnarly city to play in, and the Player's Gazetteer for all the gods, classes, timelines etc. I'll never need another setting.
Crucible of Freya. 1st level campaign kicker-offer. Used it three times since its release and it hasn't disappointed yet. For 3x, I think it is a classic that occupies the same spot as Keep on the Borderlands did for the old edition.
Of course, you can't have a Necromancer collection without Rappan Athuk. I beleive it is their signature product. (though I didn't pick up the newer reloaded box set...maybe I better get on that)
