Slaves of Heaven

British paperback edition of this 1975 novel. "Berry, chief of his clan, knew his people could survive the dangers of the forest; and when winter came he made them build barricades against raiders from other clans. But no barricades were strong enough to hold against the Night Comers - huge silver beings of horrifying strength who carried away the womenfolk and were drastically lowering the human population. Were the Night Comers men, monsters or gods? Berry believed they were man; and when the inevitable night came when the women of his clan were seized, he managed to follow. He followed them to a huge tapering column of metal, which took him away from the world he had known to an island in the sky called "Heaven." And there Berry realized that he had to defeat the Lords of Heaven if the people on Earth were to survive."