The Jewel and the Key — Louise Spiegler

An earthquake and the discovery of a mysterious antique mirror unleash forces that jolt sixteen-year-old Addie McNeal back to 1917 Seattle, just as the United States is entering World War I. Addie finds herself shuttling back and forth between past and present, drawn in both times to the grand Jewel Theater. In both decades the existence of the Jewel is threatened and war is looming . . . and someone she cares about is determined to fight.

Eventually, Addie realizes that only she has the key to saving the Jewel -- and the lives of her friends. But will she figure out how to manipulate the intricately woven threads of time and truly set things right?