Devil Red by Lansdale Joe R.
If there's one thing Hap Collins and Leonard Pine like it's trouble - and they especially like getting paid to find it. So when their friend and sometime boss Marvin Harmon asks the boys to look into a cold-case double homicide they're happy to oblige. It turns out that both victims were set to inherit some serious money and one of them ran with an honest-to-goodness vampire cult. The more closely Hap and Leonard look over the crime scene photos the more trouble they see. The image of a red devil's head is painted on a tree. A little research turns up a slew of murders with that same fiendish signature. And if things aren't weird enough Leonard has taken to wearing a deerstalker cap...Will this be the case that finally sends Hap over the edge?