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, 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.
Sandy and Dennys have always been the normal, run-of-the-mill ones in the extraodinary Murry family. They garden, make an occasional A in school, and play baseball. Nothing especially interesting has happened to the twins until they accidentally interrupt their father's experiment.
Then the two boys are thrown across time and space. They find themselves alone in the desert, where, if they believe in unicorns, they can find unicorns, and whether they believe or not, mammoths and manticores will find them.
The twins are rescued by Japheth, a man from the nearby oasis, but before he can bring them to safety, Dennys gets lost. Each boy is quickly embroiled in the conflicts of this time and place, whose populations includes winged seraphim, a few stray mythic beasts, perilous and beautiful nephilim, and small, long lived humans who consider Sandy and Dennys giants. The boys find they have more to do in the oasis than simply getting themselves home--they have to reunite an estranged father and son, but it won't be easy, especially when the son is named Noah and he's about to start building a boat in the desert.
An Acceptable Time, the final book in Madeleine L'Engle's Time Quintet, the series that began with the Newbery Award winner A Wrinkle in Time
A flash of lightning, quivering ground, and, instead of her grandparents' farm, Polly sees mist and jagged mountains -- and coming toward her, a group of young men carrying spears.
Why has a time gate opened and dropped Polly into a world that existed 3,000 years ago? Will she be able to get back to the present before the time gate closes -- and leaves her to face a group of people who believe in human sacrifice?