The series is Urban Fantasy/Noir Detective.
Abraham Grim (girl) is the go-to paranormal private detective in Manchester (UK) because she is immune to magic and can't be glamoured by vampires, nor do spells work on her. She has a part of her head shaved where she has a tattoo of an ouroboros which heats when someone is trying to spell/glamour her since there's no other way for her to know.
When Abe stubbles on four girls brutally murdered in a blood-drenched pentagram, she thinks it’s about the worst thing she’s ever seen. Until their mutilated corpses come back to life as ghouls and she has to decapitate them.
With the help of a witch, Abe follows the clues and trail of dead bodies. She discovers the girls were involved in a plan to expose a scheme perpetrated by the Humane Society—a group who work to undermine the standing of werewolves/vamps/witches—to award city contracts to human only companies.
Abe identifies the murderer and instead of calling the police, she lets the families of the murder victims deal with him.