One Step Too Far, Lisa Gardner

Five men head into the woods for a bachelor party weekend.
Only four return. If he never left the woods, where did he go?
A young man disappears during a stag weekend in the woods.
Years later, he's still missing.
But his friends who were with him that day are still searching for him. Still hunting for answers.
They hike deep into the wilderness.
With them is missing person specialist Frankie Elkin.
What they don't know is that they are putting their own lives in terrifying danger, and may not come back alive ...