Two new murder mystery novels let readers into hidden worlds: one underground and the other among the wives of serial killers. First, Ruby Falls begins in 1928 in Chattanooga, Tennessee when a man discovers a mysterious underground cavern and waterfall. In today’s episode, NPR’s Scott Simon speaks with Gin Phillips about the publicity stunt that sets her story in motion. Then, Lizzie Pook chats with NPR’s Ayesha Rascoe about The Secret Lives of Murderers’ Wives – published under the pen name Elizabeth Arnott…..