Devil's Tower — Mark Sumner

The turning point of the Civil War had come when the bodies got up at Shiloh. Dangerous magic had risen on a flood of blood and violence, and it swept across the land, washing away all but the strong and the lucky. Unnatural powers had been loosed, and nothing would ever be the same again.
Towns out west were dying faster than mayflies in June. To survive the perils of the frontierbandits, hexes, marauders, and conjurations, folks needed a strong sheriff. And any lawman who expected to survive had better have a fast gun and a talent for magic.
Jake Bird wasn't aiming to pin any star to his chest. He had a talent, maybe a strong one. But before he could search out his future, he'd have to face his past--and stand up against the man who'd killed his father. That would mean a showdown against the most dangerous wielder of magic in all the West: General George Armstrong Custer!