I have AEGs Undead, Slayers Guide to the Undead, and Necromancer's Legacy, and all three of the books are very solid IMO. I guess it depends on what you are looking for. If you want a general undead reference, with some new spells, presteige classes, alternate tactics for undead, and hints for using undead in a campaign, then AEG's Undead is probably the way to go. If you want lots of solid crunchy bits like new undead, spells, feats, and presteige classes, then Necromancer's Legacy is good. And the Slayers Guide to Undead is excellent as well- finally somone made every undead in the MM a template!!! The Slayers Guide also has lots of adventure hooks, some new undead types, and some cool new ways to use undead in a game. All three are worth the money, so it just depends on what you are looking for.