The Dark Symphony is a novel written by Dean Koontz. It was first published in 1970.
Description[]
Alliance Against The Stars!
Men came home to Earth, home from the stars...home to rule a world that they hated! But Earth was easy prey, for there was little left after the last of the atomic wars, except for pathetic mutants picking a living in the ruins...and others, creatures no longer even remotely human, who threatened to supplant the last strains of real man.
The men from the stars moved in, bringing their star-born societies, setting themselves up as masters over the mutant world...a world of creatures not even fit to be slaves! But the mutants were still there, too many to kill off, and the new races plotted together against the masters from the stars!