On the Dark Edge of Heaven. Booger McClain Ozarks Detective Series, #4

The city of Hannibal, Missouri, is steeped in history. The boyhood home of Mark Twain, this sleepy town on the western banks of the Mississippi River, holds many stories and more than a few secrets. Nearly 50 years ago, three boys went to explore Hannibal's famous caves and mysteriously vanished. The case was never solved and eventually became part of the town's folklore. Detective Booger McClain is brought in by a client hoping to bring new life to the case, and what the private investigator learns is more than either of them knows how to handle.
You see, caves hold secrets. Bizarre, dangerous, and other-worldly secrets. Detective McClain thinks he knows what happened. Now, the story just needs to be told.