The Girl with the Red Balloon — Katherine Locke

When sixteen-year-old Ellie Baum accidentally time-travels via red balloon to 1988 East Berlin, she's caught up in a conspiracy of history and magic. She meets members of an underground guild in East Berlin who use balloons and magic to help people escape over the Wall -- but even to the balloon makers, Ellie's time travel is a mystery. When it becomes clear that someone is using dark magic to change history, Ellie must risk everything -- including her only way home -- to stop the process.