The Last Child by John Hart
One year ago, 12 year old Johnny Merrimon's twin sister went missing. Never to be seen or heard from again. Johnny's father took off. Johnny's mother made the quick decent into an impenetrable depression clouded by pain killers. And Johnny has spent every waking minute tracking local pedophiles, searching the woods, and looking for clues.
Now, as another young girl goes missing in the same small town, Detective Hunt is more determined than ever to catch who is responsible and must rely on Johnny's help to lead him in the right direction.
I thought this was an excellent story and look forward to reading more by this author.