A Swiftly Tilting Planet (Time Quintet #3) by Madeleine L'Engle
In this companion volume to "A Wrinkle In Time" (Newbery Award winner) and "A Wind In The Door" 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.
But in overcoming the challenges, Charles Wallace must face the ultimate test of his faith and will, as he is sent within four people from another time, there to search for a way to avert the tragedy threatening them all.