A Swiftly Tilting Planet

In A Swiftly Tilting Planet by Madeleine L'Engle, a companion to the Newbery Award winner A Wrinkle in Time and A Wind in the Door, the Murry and O'Keefe Families enlist the help of the unicorn, Gaudior, to save the world from imminent nuclear war.

Fifteen-year-old Charles Wallace and the unicorn Gaudior undertake a perilous journey through time in a desperate attempt to stop the destruction of the world by the mad dictator Madog Branzillo. They are not alone in their quest.
Charles Wallace's sister, Meg—grown and expecting her first child, but still able to enter her brother's thoughts and emotions by "kything"—goes with him in spirit. Charles Wallace must face the ultimate test of his faith and his will as he is sent within four people from another time, there to search for a way to avert the tragedy threatening them all.
Books by Madeleine L'Engle
A Wrinkle in Time
A Wind in the Door
A Swiftly Tilting Planet
Many Waters
An Acceptable Time
A Wrinkle in Time: The Graphic Novel by Madeleine L'Engle; adapted & illustrated by Hope Larson: A graphic novel adaptation of Madeleine L'Engle's ground-breaking science fiction and fantasy classic.
Intergalactic P.S. 3 by Madeleine L'Engle; illustrated by Hope Larson: Visit the world of A Wrinkle in Time in this standalone story!
The Austin Family Chronicles
Meet the Austins (Volume 1)
The Moon by Night (Volume 2)
The Young Unicorns (Volume 3)
A Ring of Endless Light (Volume 4) A Newbery Honor book!
Troubling a Star (Volume 5)
The Polly O'Keefe books
The Arm of the Starfish
Dragons in the Waters
A House Like a Lotus
And Both Were Young
Camilla
The Joys of Love