Weird, Weird West, The (Strange Matter #16) by Engle & Barnes

A monster is coming after Shane Reece. An earthquake two hundred miles from Fairfield has unleashed something terrible - Clayton T. Motley, a zombie gunslinger looking for revenge. Back in the middle 1800s, one of Shane's ancestors ran Motley out of Fairfield by putting a bullet in his hide. A posse chased the fleeing bandit for weeks until Motley holed up in a cave, where he eventually succumbed to his gunshot wound. The bullet inside him, however, was made out of something . . . strange. Now, almost one hundred and fifty years later, the earthquake has freed the undead gunfighter from his rocky prison - and he wants revenge. He's coming after the only Reece left in Fairfield - Shane. . . . and this town's not big enough for the both of them.