I am a bonifide Necromancer addict. I can't get enough of them when I am playing and I love to include them in my campaigns when I am DM'ing. I love everything from the insane animator to the grim cleric of death to the refined master of negative energy (and every thing in between

) Now, I would like to think that I have a pretty complete list of items that deal with Necromancers:
2nd Ed
Lords of Darkness - this is a really great book, but not really necessary for you to have. Just has a bunch of nice adventures involving the undead with a story at the beggining of each chapter and some great spells at the end.
The Complete Book of Necromancers - this one is the original bible of necromancy for 2ed and I still recommend it for anyone if you can find it. Lots of background, ideas, corruptions, spells, etc. Many ideas you find in 3ed stuff came from this book.
Van Richtens guide to ....... - Vampires, LIches, Ghosts, mummies, etc. Some cool stuff in these with lots of ways to flesh out your creatures and make each one unique. The rules are too Ravenloft and 2ed and need some heavy conversion to use (if you like that sort of thing)
Now on to the Meat and Potatoes, 3rd ed
Lords of Darkness: Vampires - pretty cool book, with some real useful vampire stuff. One thing I don't like about this and Liches is that they are kinda world specific. LoD has made this setting and a bunch of the rules for the Vampires are setting specific.
Lords of Darkness: Liches - same as above but even moreso. Much of the stuff for liches is almost useless unless you are planning on running the LoD metaplot. It does have some really cool ideas though, if you go for that sort of thing.
Hollowfaust: While this entire city is set into the Scarred Lands game world, it kicks much ash (pun intended, the city is set into a volcanoe

). Great flavor, some really cool spells, magic items, prestige classes, templates, and new monsters. You won't be disssapointed, and will probably be like me and try to find a place for this city in your own campaign world.
Van Richtens Guide to the Walking Dead - This book is THE way to kick your zombies up a notch or two. Again some of this is Ravenloft specific, but it easily transports over to any 3e game.
Slayers Guide to the Undead - If you are looking for some more indepth features of the basic undead (gouls, ghasts, wraiths, vampires, ghosts, wights, liches, mummies, alip, Devourer, etc) with templates to boot then get this, otherwise its kinda vanilla.
Secret College of Necromancy - Lotsa spells, with a full 20 level Necromancer core class. Lotsa feats, lotsa magic items. Some of the spells are just too much and should be epic IMO (like the legion spells). This one is a great book to get (and one of the first in 3e, I think).
Encyclopedia Arcane: Necromancy, Beyond the Grave: (whew! long title) This one has some feats and spells, all in all its good, as said elsewher ein this thread.
Arcane Mysteries: Necromancer's Legacy - This was originally done in PDF as Gar'Udok's Compleet Librum ov Necromantic Artes. It was put to print by MEG and I would say is a must have. The spells are some of the most original I have seen and the backstory (which is interspersed amongst the material in the book) draws you in. Some of the creatures like the poison golem, and cairn worm are something you can't wait to throw at your players!
Anubium: Dark Arts of Necromancy - this one is a late comer to the world of 3e Necromancy, but has some new spells not seen elsewhere and some nicely done magic items.
Legends & Lairs: Necromantic Lore - this is primarily a book on monsters, all undead or related. I thought the Negative Energy Elemental was nice.
Races of Renown: Fang & Fury - another Vampire player book, has lotsa "blood feats" that can be taken to enhance the vampire and give it unique powers (I like this one better than LoD: Vampires).
and finally, Book of Vile Darkness. It has some new Necromantic spells and just some disturbing stuff to give to your villian. I would also recommend Relics and Rituals I & II for the spells and magic items in them. There is lots of good stuff for every kind of spellcaster (including necromancers of course).
hope that helps, though its not a complete review by any means and you should check out the reviews of these products provided by EnWorld before purchasing!
Xyth