Witch's Canyon (A Supernatural Novel) by Jeff Mariotte

What's on the back of the book:
Sam and Dean have set out on a road trip to the Grand Canyon, but this is no vacation for the brothers. On a stretch of deserted ranchland just beyond the canyon's stunning vistas, mysterious murder sprees have occurred every forty years. The areas inhabitants have been few and far between in years past, but a nearby mega-mall is about to celebrate its grand opening - and attract thousands of fresh victims.

The Winchester boys are determined to protect locals and shoppers alike, but they never anticipated they'd be fighting a group of killers this vicious, this vindictive, this . . . dead. A deadly horde of animal spirits and human ghosts has arisen to terrorize this tiny corner of the Arizona desert. If Sam and Dean can't figure out why, the wide-open spaces of the West will once again become a desolate frontier . . . and the witch's canyon will be the brothers' final resting place.

A piece from the book:
The wail of a siren jerked Dean out of a deep sleep. Cedar Wells had been so quiet, they might have been camping a hundred miles away from the nearest other humans, instead of sleeping in a motel at the edge of town. In contrast, the blaring siren was almost deafening.
"That's not good," Sam said. He slipped out of his bed and started dressing.
"A siren is pretty much always bad news for someone," Dean agreed.
By the time they made it to the Impala - a gist from Dad, 1967, midnight black, newly rebuilt - the siren had faded into the distance. But they knew its direction of travel, through town and toward the Grand Canyon. Another couple of minutes later they could see flashing roof lights flickering through the trees up ahead.
Dean and Sam got out of the Impala and hurried to a driveway that led to a big white barn. A pickup truck was parked in the driveway, and beside it was the body of what must have been a man, probably not to long ago. The truck's driver side door hung open. Blood had spattered up the side of the truck and onto the driver's seat, and the man's arm was hooked up over the step, but his throat was gone, along with the bottom half of his face, and something had opened his chest cavity. It looked like whatever had done that had been hunting for tender morsels, but Dean didn't spend a lot of time counting organs. He glanced long enough to estimate the damage, then looked away, sickened by the sight.
You could see a lot of carnage without ever growing to like it.