The Secret of Devil's Cave — Jennifer Hale

The grave was new, with a large but simple stone and an almost solid blanket of flowers on the low mound of freshly turned earth. There was no clue to the identity of the person buried there, however, for the stone was blank. And as Beth Nolan studied the photograph of the grave that had been hidden with her dead father's most important papers, she had no way of knowing that she was taking the first step along a path that led straight into horror. For nothing about her past, her family, was as Beth lead always believed, and note she must unravel the secret of that grave -- or face death herself !

Takes place in the Ozark Mountain in Missouri