Ghost Valley (Last Gunfighter Series #3) by William W. Johnstone
Outlaws have taken Frank Morgan's son, and with all the good gunfighting men either dead or dying, Morgan knows he'll be riding after the kidnappers alone. But just as he gets close to the men he's hunting, he comes upon a ghost town nestled into the Rocky Mountain valley. For Morgan, the mystery of what happened to the town - and of the deadly spirit that haunts it - has to take second fiddle to what's brought him this far. Determined to free his son, he'll lure his enemies to this godforsaken place, where amidst the ghosts, the gunfighters, and the gunsmoke, hell make sure the killing is real.