Many Waters (Time Quintet #4)
Author: Madeleine L'Engle
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.