Ghost Slayer Series by Majanka Verstraete

Twenty-one-year-old Kaelyn has spent half her life hunting ghosts and killing them. But she's not like the other ghost hunters who have to rely on spells and curses to banish ghosts back to where they came from, hoping that they don't come back. When Kaelyn kills a ghost, they stay dead. But in Mortimer Hall, a behemoth of a house, Kaelyn is about to face the most powerful and life-threatening ghost she ever met, and what she doesn't know is that the ghost has been waiting just for her