Underground. Number 3 in series

Harper Blaine was just an average small-time private investigator until she died - for two minutes. Now Harper is a Greywalker, walking the thin line between the living world and the paranormal realm. And her new abilities are landing her all sorts of strange cases. In the cold of winter, Pioneer Square's homeless are turning up dead and mutilated, and zombies have been seen roaming the streets of the underground - the city buried beneath modern Seattle.
When Harper's friend Quinton fears he may be implicated in the deaths, he persuades her to investigate their mysterious cause. But when Harper turns to the city's vampire denizens for help, they want nothing to do with her or with the investigation. For this creature is no vampire. Someone has unleashed a monster of ancient legend upon the Underground, and Harper must deal with both the living and the dead to put a stop to it .