Philippa Gregory 
Changeling 

Dark myths, medieval secrets, intrigue, and romance populate the pages of this first in a four-book teen series from the #1 bestselling author of The Other Boleyn Girl.The year is 1453, and all signs point to it being the end of the world. Accused of heresy and expelled from his monastery, handsome seventeen-year-old Luca Vero is recruited by a mysterious stranger to record the end of times across Europe. Commanded by sealed orders, Luca is sent to map the fears of Christendom and travel to the very frontier of good and evil. Seventeen-year-old Isolde, a Lady Abbess, is trapped in a nunnery to prevent her from claiming her rich inheritance. As the nuns in her care are driven mad by strange visionswalking in their sleep and showing bleeding wounds, Luca is sent to investigate and driven to accuse her. Forced to face the greatest fears of the dark ageswitchcraft, werewolves, madness Luca and Isolde embark on a search for truth, their own destinies, and even love as they take the unknown ways to the real historical figure who defends the boundaries of Christendom and holds the secrets of the Order of Darkness.