Underground (Greywalker‪)‬

The third book in Kat Richardson's best-selling Greywalker series, Underground features heroine Harper Blaine, walker of the shadowy line between the living and the dead. Seattle's homeless are turning up dead and mutilated, and zombies have been seen stalking the Underground. Harper sets out to find the source, and soon comes to realize she's tangling with an ancient evil.