poltergeist (greywalker2) by Kat Richardson

Greywalker (2006), Richardson's debut novel, introduced the character of Harper Blaine, a private investigator who, after having been clinically dead for two minutes, now straddles the line between the living and the dead. With the ability to pass back and forth between this world and the Grey world—where monsters are real, and evil is not an abstract concept—she is uniquely equipped to handle cases with paranormal connections. Here, Harper is hired to find out who has been faking results in a university-sponsored investigation into poltergeists. When (naturally) someone turns up dead, Harper must determine whether the killer is human or something else.