The Coldest Girl in Coldtown, Paperback

Coldtown was dangerous, Tana knew. A glamorous cage, a prison for the damned and anyone who wanted to party with them.
Tana lives in a world where walled cities called Coldtowns exist. In them, quarantined monsters and humans mingle in a decadently bloody mix of predator and prey. And once you pass through Coldtown's gates, you can never leave.
One morning, after a perfectly ordinary party, Tana wakes up surrounded by corpses. The only other survivors of this massacre are her exasperatingly endearing ex-boyfriend, infected and on the edge, and a mysterious boy burdened with a terrible secret. Shaken and determined, Tana enters a race against the clock to save the three of them the only way she knows how: by going straight to the wicked, opulent heart of Coldtown itself.