Two Documentaries that jump to mind: PBS' "The Circus" is a REALLY good intro to the history of the circus as an American institution. Barnum, of course, features heavily, and much time is devoted to him, his rivalry with Adam Forepaugh for the mastery of the industry, his later partnership with James Bailey (who Zac Efron's character in Greatest Showman is partially based on), and the eventual rise and triumph of the Ringling Brothers. Secondly, 1995's "P.T. Barnum: America's Greatest Showman" is a more specific look into Barnum in particular (as well as what first got me and my sister interested in the guy). There's obviously overlap in material between the two programs, and "The Circus" is probably going to be easier to find in high def. "America's Greatest Showman" is also based on a book by Philip and Peter Kunhardt that goes into even MORE detail about him (especially his time in politics). And of course, you could do a lot worse than PT Barnum's own autobiography (now published under an array of titles, but not at all hard to find). As I pointed out, he was remarkably candid about what sort of guy he was and what sort of business he was running.
