Seeker by Arwen Elys Dayton

Seeker is a nail-biter of a book, full of action, hints of romance, and agonizing moral choices as three teens, Quinn, Shinobu, and John, struggle for control of time- and place-bending powers.  Quinn and Shinobu, upon taking their oaths as seekers, discover the darker side of their new responsibilities and want nothing more to do with them.  Escape is seen as betrayal and carries its own consequences, and John's desire to restore his family's legacy ultimately threatens to destroy Quinn, even though he loves her.  Great stuff!!