Ghost Detectives

I'm currently reading "Haunted" by Kelley Armstrong, which is book 5 of 
the Women of the Underworld series.  In this book, long dead Eve, a 
black witch is called to repay a favor to the fates by looking for a 
demi demon that is possessing women and causing them to be killers. 
It's a great book, and it's got me thinking.

The idea of a ghost on the "other side" solving mysteries/hunting down 
souls etc is an intriguing one to me.  Do any of you know of any series 
like this?  If not, there should be...