Ghostwalk is the type of product which either makes you go "eh", or changes every campaign you will ever DM again. I am definitely in the latter group. Ghostwalk found me at just the right time, and filled a need in my developing campaign that I'd just begun to consider.
The concept, in a nutshell, is that when your character dies, he or she can continue adventuring as a ghost. You can continue to gain experience and advance in one of two ghost classes, eidolon and eidoloncer. The eidolon advances attack and save bonuses, and allows you to take quite a few new ghost feats. Eidoloncers trade rapid accumulation of ghost feats for advacing their spellcasting abilities. Ghost feats are grouped into several paths, representing traditional ghostly powers of corruption, possession, poltergeists, etc. The rules seem well considered and tested, and the ghost path feats simultaneously cover all the traditional ghostly powers while leaving a great deal of room for imaginitive DMs to expand upon.
Some ghost related skill uses, feats, and prestige classes are provided, some original and some drawn from other campaign settings or source material. This isn't the highlight of the book, but serves to illustrate how the rest of the world relates to ghosts. Some may gripe at the significant amount of material duplicated from other sources, but first not all DMs will own all the sources in question, and second its indisputably handy to have the relevant materials in a single volume regardless.
Ghostwalk also presents a campaign setting, the city of Manifest. I haven't delved into this section of the book as much, since I'm not able to use it in my own campaign, but it looks good. The setting makes effective and original use of the ghost concepts and rules, and could likely be dropped into most campaigns readily enough. A map and description of the city itself is provided, as well as descriptions of the surrounding lands and cultures. A map of the region was accidentally dropped from the book, but see Sean K. Reynolds' website. Several short but well developed adventures in Manifest round out the section.
Finally, no setting is complete without a few new monsters.