Nemesis - The Alliance Series, #2

It's hard to defend the Earth from deadly monsters when you suspect you might be one of them. Discovering she's a walking magical weapon is just the beginning of Ada's problems. Her guardian thinks she's turned traitor, and her new boss at the Inter-World Alliance has put her on goblin-catching duty. With her family's livelihoods at stake, Ada must cooperate with the Alliance despite their obvious interest in exploiting her magical talent. Kay, meanwhile, is thrown into the middle of political turmoil on a world split between humans and centaurs, as the centaur king's murder threatens to trigger a war with their magic-wielding human neighbours.
To prevent a bloodbath reaching Earth's doorstep, Kay must help Markos find the real killer -- and learn more about his own unpredictable magic in the process. It isn't long before Ada and Kay's paths collide again as they face a conspiracy that could change the future of the Alliance.