One Step Too Far by Lisa Gardner

Frankie Elkin, who readers first met in Before She Disappeared, learns of a young man who has gone missing in a national forest. Law enforcement has abandoned the search, but a crew of people led by the young man’s father are still looking. Sensing a father’s desperation, Frankie agrees to help–but soon sees that a missing person isn’t all that’s wrong here. And when more people start to vanish, Frankie realizes she’s up against something very dark–and she’s running out of time.