Prisoner of Time — Caroline B. Cooney

Tod Lockwood has never wanted to be anyone's knight in shining armor. In fact, he wants to avoid having anything to do with girls, at least for the present. But that's before Devonny Stratton steps into his life out of the nineteenth century.

As for sixteen-year-old Devonny, she has no plans for marriage until her father arranges to wed her to the contemptuous but well-connected Lord Winden. Devonny has only one hope. Someone must rescue her. Can Tod Lockwood be Time's answer to her prayers? Life never seems simple to Devonny, but do the solutions to her problems await her in the future? Or will she only become a prisoner of a different time?