Wild Magic - The Gatekeeper's Fate, #1

Being the villain is easy. Being heroic is a thousand times harder. Once, Holly Lynn was the Winter Gatekeeper, wielder of the powerful magic of the Unseelie Court. Now, she's shunned by humans and faeries alike, reduced to hunting troublemaking fae beasts to pay the bills and trying to convince her landlord not to kick out the traumatised teenage death faerie who's moved into her room. When a serial killer starts picking off the local faeries, Holly's hopes of lying low are shattered.
A local half-fae detective takes an interest in the case, and he seems to think Holly can help him solve it. The slight problem? Puck is a descendent of the notorious Robin Goodfellow, the master prankster from the Summer Court and sworn enemy of the Unseelie. If he finds out Holly's past, she can say goodbye to keeping her former vow to the Winter Court a secret. With the clues to the killer's identity pointing directly to Faerie itself, though, Holly finds herself in a dilemma.
She knows better than to make bargains with the fae, but to solve the murders and stop the killer's rampage, she might have no choice in the matter.