Nancy Drew Mystery Stories, The Secret of Red Gate Farm, Book 6 by Carolyn Keene

Nancy and her friends, Bess and George, meet Joanne Byrd on a train ride home. Joanne lives at Red Gate Farm with her grandmother, but if they do not raise enough money to pay the mortgage, they will soon lose the farm Nancy, Bess, and George decide to stay at Red Gate for a week as paying customers. Soon, they learn about the strange group of people who rent a cave on the property. They describe themselves as a nature cult called the Black Snake Colony. Nancy investigates their group and helps to uncover a ring of counterfeiters in town.