Elite by Mercedes Lackey

Joy knows she'll be facing more dangerous Othersiders than ever before as a new member of the Elite Hunter unit, but if anyone is up to the challenge it's her. She's been Hunting these monsters since she was a child, and has a pack of eleven fiercely protective magical Hounds. Then the rules change. Monsters unlike any Joy's ever seen or even heard of are breaking through Apex City's barriers and the Hunters are scrambling to find new ways to fight them--all the while hiding the true danger Apex faces from the Cits, who are ignorant of the severity of the Othersiders' attacks.
The leaders of Apex must come together to protect the city, but tensions have never been higher between the Hunters and the powerful PsiCorps, with each group competing to be the primary protector of the city. The conflict escalates even further when Joy starts discovering bodies of Psimons while patrolling the city sewers--on a special assignment from her uncle, who commands the Hunters. Someone is killing Psimons and if Joy doesn't uncover the true culprit she might just take the fall for it.
Chaos erupts when Ace, the murderous Hunter who tried to kill Joy at her Elite trials, escapes from the Army's captivity and defects to the Othersiders. Joy has no idea what Ace might be capable of with the help of the cunning Folk, but she may be about to find out; Othersider forces are gaining strength and momentum just beyond the barriers. A storm is approaching Apex City, and unless Joy and her fellow Hunters put up the fight of their lives it might just sweep them all away.

This book is part of the "Hunter" series.
Here are some other books from this series: