D. W. Griffith based his film on Thomas Dixon's sensational, successful novel The Clansman (1905) and the subsequent play. See Jeffrey B. Martin, "Film Out of Theatre: D. W. Griffith The Birth of a Nation and the Melodrama The Clansman.” Literature/Film Quarterly 18.2 (1990): 87-95. See also Aaron Baek's source essay here on Reel American History.