During the first half of the 20th century, radio's hunger for captivating characters and stories could not be sated. Three national networks and dozens of independent stations had to fill a vast expanse of air time with comedy, adventure, mystery, drama and music, night after night. It's no surprise that producers and writers looked to outside sources, drawing some of old-time radio's most beloved characters (Sherlock Holmes, Tarzan, Hopalong Cassidy, Buck Rogers) directly from books....