A Swiftly Tilting Planet — Madeleine L'engle

Meg Murry O'Keefe and her family are just sitting down to Thanksgiving dinner when her father gets a phone call from the White House about a madman's threat of nuclear war. Only an old Irish rune seems to hold a clue to averting worldwide disaster. When Meg's brother Charles Wallace, now fifteen, recites it, a radiant white beast -- the unicorn Gaudior -- appears, to join him on his quest. But there are only twenty-four hours in which to prevent the tragedy. Can Charles Wallace, with the help of Gaudior and Meg, possibly succeed?