The Bone Riders. The Bone Riders, #1

As a Bone Rider, Drew Harlow is no stranger to death. For centuries, the Bone Riders have guarded the rifts, places where magic enters the world, to rectify the incomplete deaths that are caused by the rifts' proximity. Drew is less than enthusiastic about her newfound death magic, but after a boring day at the office, sometimes even patrolling for roadkill with her Welsh pony, Charon, seems more interesting. When a surge in the magic of the rift beneath Asterville causes the dead to become active in ways the Riders have never seen before, Drew sets out to investigate the source of the strange magical phenomena taking over the city.
Her search leads her to the wider world of magic users, but the shadow of someone working against the Riders lurks around the edges of the graveyards, and Drew's nosiness just might get her in trouble.