SF. The Dark Symphony - Dean R. Koontz, paperback as description

Men came home to Earth, home from the stas, 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.

A Lancer 1970 paperback in very good to fine condition based on the Hancer Standard.