Title:A Swiftly Tilting Planet
Author: Madeleine L'Engle
Source/Format: Won, Paperback Omnibus
More Details: Middle Grade, Science Fiction, Fantasy
Publisher/Publication Date: Square Fish, July 1978
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Synopsis from Goodreads...
When fifteen-year-old Charles Wallace Murry shouts out an ancient rune meant to ward off the dark in desperation, a radiant creature appears. It is Gaudior, unicorn and time traveler. Charles Wallace and Gaudior must travel into the past on the winds of time to try to find a Might-Have-Been - a moment in the past when the entire course of events leading to the present can be changed, and the future of Earth - this small, swiftly tilting planet - saved.
A Swiftly Tilting Planet is the third book in Madeleine L'Engle's classic Time Quintet...