Sincerely, From the Other Side by Agnes Rooke
When Claudia befriends a mysterious fellow in the college library, she finds herself in a world where spirits dress up as people, and vacations means travel between the living realm and limbo. As the new exorcist in town, it’s her job to find approximately 300 missing ghosts, but as Claudia learns, there’s more to the Spirit World than bad puns and daisy-patterned sheets. At least one of her new friends is lying about their intentions and some of the more vengeful spirits are just dying for her permanent companionship.