Second Harry Dresden novel. It opens with Harry earning his dinner by looking at something an acquaintance (who is not quite given enough intro) shows him. It's a magical circle. And he warns her off. Whatever she wanted to deal with using that, she didn't want to deal with it if she had any sense. She is not pleased.
Then he gets called in by the cops, even though Murphy is having trouble with her bosses and hiring him is one reason. He confirms that it looks like a werewolf kill before the FBI throws them both out. And he goes to consult with Bob, his talking skull, to learn about werewolves. Bob, in fact, enlightens him about four types.
The murders -- and the circle -- are, it turns out, related along with vigilante justice and scapegoats for it, belts of fur, inherited silver, a werewolf tearing through the police station, a local crime boss trying to hire Harry and Harry's asking for the pen, and much more.
Reflections from reading ahead:[Spoiler (click to open)]His relationships with Murphy and Susan still are just beginning to develop. And only with hindsight can you see where he's starting to pick up friends here.