The Iron Heel is interesting as an example of a "dystopian" novel which anticipates and influenced George Orwell's Nineteen Eighty-Four. Jack London's (1876-1916) socialist politics are explicitly on display here.
Its description of the capitalist class forming an organised, totalitarian, violent oligarchy to crush the working-class forewarned in some detail the Fascist dictatorships of Europe. Given it was written in 1908, this prediction was some-what uncanny, as Trotsky noted while commenting on the book in the 1930s.