Trail of Dead — Melissa F. Olson

As a null, Scarlett Bernard possesses a rare ability to counteract the supernatural by instantly neutralizing spells and magical forces. For years she has used her gift to scrub crime scenes of any magical traces, helping the powerful paranormal communities of Los Angeles stay hidden. But after LAPD detective Jesse Cruz discovered Scarlett's secret, he made a bargain with her: solve a particularly grisly murder case, and he would stay silent about the city's unearthly underworld.

Now two dead witches are found a few days before Christmas, and Scarlett is once again strong-armed into assisting the investigation. She soon finds a connection between the murders and her own former mentor, Olivia, a null who mysteriously turned into a vampire and who harbors her own sinister agenda. Now Scarlett must revisit her painful past to find Olivia -- unless the blood-drenched present claims her life first.