Glass, book 3: Spy Glass
Maria V. Snyder2010

An undercover mission leads to danger, adventure and an impossible choice.

After siphoning her own blood magic in the showdown at Hubal, Opal Cowan has lost her powers. She can no longer create glass magic. More, she's immune to the effects of magic. Opal is now an outsider looking in, spying through the glass on those with the powers she once had, powers that make a difference in the world. Until spying through the glass becomes her new power. Suddenly, the beautiful pieces she makes flash in the presence of magic. And then she discovers that someone has stolen some of her blood - and that finding it might let her regain her powers. Or know it could be they are lost forever.