Captured in Crystal — Melanie Card

The Amber Orb's magical bond is unbreakable and the only escape is death.

And Thea needs to be free of her bond. Her partner, Brand, is a Knight Blade too handsome for his own good and rumored to have murdered his previous partner. He's the last person she wants to spend the rest of her life with, but Thea is a Knight Key of the Amber Order--even if she's never manifested any magical abilities--and has sworn an oath of obedience.

With no time to deal with her new partnership and her attraction to Brand--surely a side-effect of the bond--they are thrust on their first mission: travel to the mysterious Crystal City with its strange, reclusive people and determine why the High King's ambassador has gone silent. But dangerous magic lies within the city's shimmering walls, magic that threatens to destroy everyone... if her bond with Brand doesn't destroy her first.