I'll get right to the review: The book is very useful, probably 60/40 flavor/function split.
-Undead Lore:
This section deals with the most flavor. It talks about the creation of undead, their physiology, mentality, and society. The most useful parts pertained to locations, especially Hantumah, a totally undead city, which seems a nice place to siege. My major gripes about this section are the attempts to mix magic and biological processes, and the fact that the author can't seem to go three paragraphs without bringing up Orcus, Demon Prince of the Undead, and how he wages a constant struggle with the very generic Raven Queen.
-DM's Guide to Undead
This might be my favorite part of the book. Right off, it gives some skill challenges, which are predictably of the 'appease this creature or else' sort, but welcome nonetheless. Then it goes into hauntings, presenting them as plot, traps, terrain, and skill challenges! Three short undead adventures are then given, culminating with advice on running a big zombie battle. A swarm of medium creatures which is also a skill challenge? Looks good to me. The chapter also gives three short campaign arcs about undead, a good quantity of pro- and anti- undead artifacts, a handful of rituals, and undead grafts.
I am taking an extra bit here to talk about undead grafts, because I feel it is a great idea that wasn't handled very well. The book explains grafts first by saying it's an unorthodox way to enhance characters in a macabre setting, but the first example is a giant flaming bone claw! Uhh? Two more examples- a graft that gives you fangs... and also a vampire's thirst for blood. The last example gives you a Lich's brain, and I have no idea how. So, grafting undead body parts? Sure. These particular ones? Nahhh.
-Undead lairs
This chapter provides 9 relatively robust, complete with map, dungeon delves, the highlight of which is a dungeon entirely inside a giant space-god-corpse. All of these look worthwhile and can be dropped into a 4th campaign quite easily.
-New Monsters
This last chapter provides a bunch of new undead, including the brain in a jar. Most all of this stuff looks good, or at least usable, and my biggest problem with this section is that one would think it would be earlier in the book, considering it is going to be used often, and referenced by earlier chapters. The undead hall of infamy is a sub-chapter here, which gives stats for beings such as Acererak, Kas, and yes, Vecna. Yes, he's LVL35. Yes, he's killable, and yes, it is a huge (campaign length) pain in the rear to do so.
-Undead Templates
This last chapter provides a handful of useful templates to be added to players or monsters, as well as a variety of special undead powers.
Overall, I give the book 5 stars because it reasonably provided everything I could ask for, undead-wise. There are a couple of issues, but these pale in comparison to the amount of useful information given.