Poltergeist by Kat Richardson
This is the second in the Greywalker series and I liked it as much, if not better than the first one. Harper Blaine has been able to enter the "grey" world since almost having died herself. The grey world is a world of echoes from past lives and times. The grey world exsists as overlapping and layered levels of our own world. In Poltergeist, Harper tracks down a killer who is using a ghost to help him carry out his crimes.