Isadora Drake and the Radio Mystery — Stephen E. Stanley

It's 1922 and Isadora Drake having solved one murder decides to open an office and offer discreet inquiries for those needing answers. Isadora's first client is a young woman concerned about the threatening letters her father has received. When she takes on the case, it's not long before developments turn a simple case into something more sinister. The new medium of radio offers clues that may help to solve the case.