Nancy Drew Mystery Stories #53: The Sky Phantom book by Carolyn Keene

Nancy goes to the Excello Flying School in the Midwest to take lessons, while her friends Bess and George perfect their horseback riding. At once the young sleuth is confronted with the mystery of a hijacked plane and a missing pilot. Then the rancher's prize pony, Major, is stolen. Nancy becomes a detective in a plane and on horseback to track down the elusive sky phantom and the horse thief. A lucky find - a medal with a message to be deciphered on it - furnishes a worthwhile clue. Romance is added to mystery when Bess becomes interested in a handsome cowboy. Readers will spur Nancy on as she investigates a strange magnetic cloud, hunts for the horse thief, and finally arrives at a surprising solution.