Correction Appended: Sept. 20, 2013
It's 10 A.M. and Dan Carlin is groaning. Stuck in bed with a bad back, he is nonetheless poring over a half-dozen thick volumes sprawled before him. Carlin, 47, is the host of the popular Internet-radio programs Hardcore History and Common Sense. In front of him lies a fraction of the research he'll eventually use to produce his next four-or five-hour-long audio broadcast, the exact subject of which is a closely guarded secret.
Carlin's brand of storytelling has garnered Hardcore History, the more popular of his shows, nearly a million listeners per episode. The program is filled...