Get Libris Mortis. I picked it up yesterday at my FLGS, and while I liked both MM III and Frostburn a lot, Libris Mortis is by far the best of the three. Lots of good fluff about undead, the condition of undeath, and their mental outlook, as well as how to run undead focused campaigns. The spells look really nasty, and the PrCs, while not something I usually care much about, seem like interesting concepts. There are also racial advancements ala Savage Species for several types of undead, and thoughts for using undead as PCs. There are also some cool ideas for undead craving life force, and what happens to their psychological state when they are starved of it.
There is a section about deities of the undead in the book, as well as undead and how they fit into other socieities. There is an interesting section on equipment and items geared towards undead, but by far the biggest part of the book is for new undead beasties. Most of the new undead types seem pretty cool, and I could definitely see myself using in a darker sort of campaign (which is what I love to run). The last part of the book presents half a dozen adventure locations with undead, the sort you can drop into an exisiting campaign (such as the House on the Hill, the Mortuary, and the Ghoul Colony). Very cool stuff. For me, this book is MUCH more useful than the Draconomicon since I tend to use undead about 100 times more often than dragons. This book, Frostburn and MM III have really impressed me- WotC might be back on the path to some really good supplements again.
