The Sky Is Falling by Lester Del Rey, Science Fiction, Fantasy, Adventure (Paperback)

Dave stared around the office. He went to the window and stared upwards at the crazy patchwork of the sky. For all he knew, in such a sky there might be cracks. In fact, as he looked, he could make out a rift, and beyond that a . . . hole . . . a small patch where there was no color, and yet the sky there was not black. There were no stars there, though points of light were clustered around the edges, apparently retreating.