The Space Sorcerers

Kill and die. That was the battle tactic of the Tinkers, rough, vicious interplanetary invaders who would rather die than be captured alive. Controlling them in some way were The Six - an occult group that had learned how to duplicate human beings and use them as pawns in a grandiose plan to control interstellar space. Defending civilization against this onslaught were Rey Cottrell, a war consultant without even a fortress from which to fight his battle, and Captain Brixby, a lone idealist faced with mutiny when he tried to take his ship into the combat zone.
Their chances seemed hopeless - especially since Cottrell suspected that there were supernatural forces involved that could take over the whole galaxy!