Circle of Stones — Catherine Fisher

Three interwoven, spine-tingling historical thrillers from the New York Times bestselling author of Incarceron.
 
Suspense, mysticism, and history encircle three separate but related narratives in this fantasy novel. Today, Sulis, a teenage girl with a mysterious past, arrives in Bath with a new identity, trailed by the person she's trying to outrun.  In 1740, Zac is apprenticed to an architect obsessed with Druidic mysteries, but has his own secret -- and destructive -- agenda.  In ancient England, a druid king discovers the healing waters of a magical spring, where he founds a great city, and the heart of Fisher's story. Through each voice, the mysteries are revealed, linking Sulis, Zac, and the king through the circles of time.