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.