The Magic Manuscript — Barbara Burgess

Two teenagers, Arthur and Jennifer, unearth a medieval manuscript in Cornwall, in modern England, during the Christmas holidays. While reading the manuscript, Jennifer becomes engrossed in the story of fifteenth-century Lady Genevieve, a sixteen-year-old Cornish girl who escapes from her brother when he tries to marry her to one of the earls of Tudor against her will. Jennifer is, in reality, catapulted back to the fifteenth century, as well as to England in the sixth century. When Arthur realizes she is missing, he follows her back in time.