This was a pretty decent introduction to this world. It's well-written and doesn't clock you over the head (too much) with the world-building. You learn only what you need to know for the story, but it still sets things up for future books and leaves room for expansion as the series goes on.
Kate Daniels is, in a lot of ways, the female counterpart to Harry Dresden. She's tough, magical, and investigates things. She's also a mercenary who is working a case pro-bono for the wizard order that her mentor belonged to - her dead mentor whose murder she is investigating. She's out for herself, but she's not selfish or arrogant. What I like about her most so far though is that she's allowed to be emotional without having her emotions define her or cloud her judgement, and she's comfortable with herself and her body even though she doesn't meet the "classic beauty" checklist requirement. She's not very good with the big picture, and she sucks at relationships. She's a well-rounded protagonist who isn't squeaky clean boring but not also a glorified anti-hero. She's complex and the main driving force of this book.
The mystery was pretty well constructed. I lost track of some of the characters at one point (long holiday weekend without any time to listen sometimes will do that), so I had to go back and relisten to sections of it and even then, knowing what the clues were, I didn't pick out the whodunit. The creature is one I haven't encountered before in my admittedly limited exposure to urban fantasy, and it was sufficiently creepy and gross.
The narrator does a great job bringing Kate to life. Her range for the male voices isn't that great, but it's good enough that I knew who was talking, and she was consistent with the voices throughout. She really puts her all into every character, and she gets the humor and emotional beats down pat.