Here's a little background on the series. In Las Vegas, the Agents of Light and the Agents of Shadow are battling to keep mankind safe. Each troop consists of agents that correspond with each sign of the Zodiac. Joanna Archer is raised human and has no idea any of this world exists until her 25th birthday when she gets her Zodiac powers. The series follows Joanna as she tries to accept her new life and responsibilities.
It takes Pettersson a little while to get her world up and running. I really enjoyed the first two books, but it wasn't until the third book in the series that everything clicked for me. Joanna Archer is a great character. She's dark and tortured. Fans of Kim Harrison's witch, Rachel Morgan, will definitely like Joanna. Of all the Urban Fantasy that I've read, this is one of my favorite series. Pettersson creates her own world rather than putting her own spin on vampires and other supernatural creatures. Her superheroes get their powers from comic books (it's a little more involved than that in the books), and it can seem a little precious, but it's almost tongue in cheek. Vegas is so prominent in the books that the City is almost like a character. I can't imagine them being set anywhere else.
City of Souls is the fourth book in the series, and it's very transitional. In this installment, Joanna must venture to Middleheaven to fix the changeling she broke. The Manuals of Light (the comic books they draw their power from) aren't being written, and her troop is weakening. A lot goes on in this book. It's big stuff for the series too. I can't spoil any of it for you, so I won't say much. I'll just say that Pettersson takes everything you know about the series and turns it on its head. The fifth book, Cheat the Grave, comes out this summer and I'm dying to see how she's going to continue the series.
If you like your heroines dark, check out The Signs of the Zodiac. They've got plenty of action, and the world that Pettersson has created is very unique. The Scent of Shadows is the first in the series. If you're already a fan of the series, make sure you check out Unbound, edited by Kim Harrison. Pettersson has a story in it called Dark Matter. However, I recommend waiting until after you've read City of Souls before you read Dark Matter. The reveal in City of Souls is more dramatic if you haven't read Dark Matter.
And just for fun... In the books, the Agents of Light enter their sanctuary through the old neon sign that used to be on the Silver Slipper in Vegas. The sign was recently restored, so I had my picture taken in front of it with City of Souls. Here's my picture.

