The Clansman

Birth of a Nation was based on Thomas Dixon's (1864-1946) 1905 novel, The Clansman.  The novel (part of a trilogy about the Klan) argues for racial purity, and the sanctity of family, as well as against socialism.  It's still in print.

Dixon's (and Griffith's) greatest fear was miscegenation (mixing of blacks and whites).  For them, Whites were whites, blacks were blacks, whites were clearly superior and the world would be happy if both understood their place and stayed in it.

Griffith's film was racist, Dixon's book almost unbelievably so. 

If you want to read a nasty short quote from Dixon, click here.