Promise Me / The Woods.

The Promise: It has been six years since entertainment agent Myron Bolitar last played superhero. In six years he hasn't thrown a punch. He hasn't held, much less fired, a gun. He hasn't called his friend Win, still the scariest man he knows, to back him up or get him out of trouble. All that is about to change ... because of a promise. Myron is determined to help keep his friends' children safe, so he makes two neighborhood girls promise him: If they are ever in a bind but are afraid to call their parents, they must call him.

The Woods: Twenty years ago, four teenagers at summer camp walked into the woods at night. Two were found murdered, and the other two were never seen again. From the cover.