Fire and thorns - Rae Carson

Princess Elisa is a disappointment to her people. Although she bears the Godstone in her navel, a sign that she has been chosen for an act of heroism, they see her as lazy and useless and fat.

On her sixteenth birthday, she is bartered off in royal marriage and shipped away to a kingdom in turmoil, where her much-older-and extremely beautiful-husband refuses to acknowledge her as his wife. Devastated, Elisa decides to take charge of her fate and learn what it means to bear the Godstone. As an invading army threatens to destroy her new home, and everyone at court maneuvers to take advantage of the young princess, Elisa becomes convinced that, not only is her own life in danger, the whole world needs saving. But how can a young girl who has never ridden horseback, never played the game of politics, and never attained the love of a man save the world? Elisa can't be sure, but she must try to uncover the Godstone's secret history before the enemy steals the destiny nestled in her core.