Giants on Troglodyte Mountain by R.V. Hodge

Exploring around Stony Mountain, three young sisters make a startling discovery. Hidden away in the mountain wilderness is a mysterious colony of Troglodytes. The sisters befriend the peculiar tribe and are sworn to secrecy to protect the reclusive cave dwellers. The girls soon learn, however, that their new friends are in mortal danger. Working together, they race to confront each threat that could be deadly for the Troglodytes. In their adventures, the Riese sisters learn the value of courage, honor, and creativity when facing unknown dangers.