Possessed — P.C. Cast

Being a psychic detective who can channel only negative emotions makes Kent Raef good at catching murderers, but bad at maintaining relationships. Then Lauren Wilcox arrives with a most intriguing case: her twin sister has been murdered and is communing with Lauren's spirit-and sharing her body. Raef's the only one who can track the killer and free the spirit. But soon he begins to wonder just which twin he wants to save...and why....