Vow of Thieves by Mary E. Pearson

Kazi and Jase have survived, stronger and more in love than ever.

Their new life now lies before them--the Ballengers will be outlaws no longer, Tor's Watch will be a kingdom, and Kazi and Jase will meet all challenges side by side, together at last.

But an ominous warning mars their journey back, and they soon find themselves captured in a tangled web of deceit woven by their greatest enemies and unlikeliest allies, a place where betrayals run deeper and more deadly than either had thought possible and where timeless ambitions threaten to destroy them both.