Into the Dim — Janet B. Taylor

When fragile, sixteen-year-old Hope Walton loses her mom to an earthquake overseas, her secluded world crumbles. Agreeing to spend the summer in Scotland, Hope discovers that her mother was more than a brilliant academic, but also a member of a secret society of time travelers. And she's alive, though currently trapped in the twelfth century, during the age of Eleanor of Aquitaine. Hope has seventy-two hours to rescue her mother and get back to their own time. Passing through the Dim, Hope enters a brutal medieval world of political intrigue, danger, and violence. A place where any serious interference could alter the very course of history. And when she meets a boy whose face is impossibly familiar, she must decide between her mission and her heart -- both of which could leave Hope trapped in the past forever.