Originally serialized in the 80's, it looks like Lansdale revised the text a it for this Nightshade Books edition from 2005. I can't knock it for not breaking any new ground, since it originates from a period long before zombies became trendy, an age when only zombie aficionados (and -phobes) spent much time on the zombie oeuvre.
Although it doesn't add much novelty, what it does do well is provide classic zombie and classic Western in a satisfying Reese's zombie western cup. The haunted preacher, the doc, the kid, the feckless sheriff, and a horde of walking dead. It starts slow and works itself up inevitably to a fast and frantic climax.
If I wanted to get hifalutin', I'd describe it in terms of the vengeance of the gods in the face of mortal transgression of xenia. But really, it's zombies in the Old West.
Parenthetically, I should note that I thoroughly enjoyed the Undead Nightmare add-on to Red Dead Redemption, which also provides a zombie western experience. I was pleasantly surprised by how much had been packed into this mini-campaign.