Ghostwalk: Campaign Option (Dungeons & Dragons Setting) 
Ghostwalk contains everything needed to run a stand-alone campaign in and around the city of Manifest, or to integrate it into an existing world, including rules for playing ghost characters and advancing in the new eidolon and eidoloncer classes, several new prestige classes, over 70 new feats and 65 new spells, three complete adventures, four highly detailed encounter sites, and fourteen new monsters and templates.
To use this accessory, a Dungeon Master also needs the Player’s Handbook, the Dungeon Master’s Guide, and the Monster Manual. A player needs only the Player’s Handbook.
Reviews
...so his friends take him to the party cleric or the local high temple and get him resurrected.
As with most of the books from Wizards of the Coast, this, too, is not needed. Anyone with a brain and a smidgen of imagination (and if you role-play, you have to have it) can change a few details of the pre-generated undead in the Monster Manual and save themselves $30.
You want innovation? Go to Green Ronin.
It is best used as a meta-setting the same way that Planescape or Spelljammer are used. Making a few assumptions helps this game truly shine and answer some of the great unanswered questions of the D&D Cosmology:
Assumption 1-
Each prime has a "ghostwalk" leading to a distinct True Afterlife maintained by the gods of their world.
Assumption 2-
Souls that "willingly discorporate" in the True Afterlife are lead into the Well of Souls and out into the Planes.
Assumption 3-
Demihumans everywhere go through the Etherial voyage mentioned in the book, however in most primes the etherial boundary is too thick for ghosts to manifest through sheer will--so they either remain stranded or embrace the seductive power of the Negative Energy Plane into themselves, becoming corrupted into undead forever, even in the True Afterlife.
This resolves a lot of questions. What's the difference between the dead and undead and why do the undead have a connection to the Negative Energy Plane?
With infinite Prime planes, why isn't the flood of souls to the Outer planes neverending?
How do dead folks get to the planes?
If Ghostwalk is it's own cosmology, why does it mention the planes and intrinsically D&D characters like Orcus?
But yeah, Ghostwalk is a lot of fun. The entries on the various cultures are awesome, the listing on the city of Manifest is great. It contains the origins of the Yuan-Ti, which by itself is reason enough to get the book. The authors really put did put some quality effort into this--the writing is very polished and the artwork is very impressive on the whole. The absence of a name for the game world is very strange, as I would like at least a suggestion of what to call the world where this all happens, but otherwise an excellent setting.