The Ladder in the Sky / The Darkness Before Tomorrow

From Author Wars - https://authorwars.com/books/the-ladder-in-the-sky--the-darkness-before-tomorrow-792293.html

Overview:

Title: The Ladder in the Sky / The Darkness Before Tomorrow
Author: John Brunner Robert Moore Williams
Year: 1962
Type: Omnibuses

Variant Titles:

1962. The Ladder in the Sky / The Darkness Before Tomorrow