Tonight, I Burn

Penny and the rest of the Thorn Coven are prisoners of the High Warden. They are forced to patrol the veil between life and death to make sure nothing gets through. In order to do that, each night one of them must burn. They are tethered to their crystals and can follow their lifeline back.
One night Penny's sister goes on patrol and doesn't come back. Penny has never burned before, but she will now so she can bring her sister back.
What she finds in death is a shocker!
I liked this, but my big complaint is the lack of back story. How they came to serve the Warden, etc. I just wanted more of an explanation for how they got there. Other than that, it was good.

Thank you to Net Galley and the publisher for an e-arc.