Jennifer Bradbury’s engrossing historical fiction, “River Runs Deep,” is set in Mammoth Cave. Elias, who is twelve years old and suffering from consumption, is ordered to dwell in the center of the biggest cave in America. Bed rest and herbal remedies may seem like an adventure at first, but they soon grow boring. Elias makes friends with Doc Croghan’s slave, Stephen, and unearths a secret buried far beneath the surface. Elias has to choose whether to put his life in danger or keep it safe while he solves the mystery. The book skillfully combines bravery, adventure, and history.