The boy from the woods / Harlan Coben.

Thirty years ago, a child was found in the New Jersey backwoods. He had been living a feral existence, with no memory of how he got there or even who he is. Everyone just calls him Wilde. Now a former soldier and security expert, he lives off the grid, shunned by the community - until they need him. A child has gone missing. With her family suspecting she's just playing a disappearing game, nobody seems concerned except for criminal attorney Hester Crimstein. She contacts Wilde, asking him to use his unique skills to find the girl. But even he can find no trace of her. One day passes, then a second, then a third. On the fourth, a human finger shows up in the mail.And now Wilde knows this is no game. It's a race against time to save the girl's life - and expose the town's dark trove of secrets...