The Girl of Fire and Thorns by Rae Carson
Once a century, one person is chosen for greatness. Elisa has always felt powerless, useless. Now, on her sixteenth birthday, she has become the secret wife of a handsome and worldly king - a king who needs her to be the chosen one, not a failure of a princess. And he's not the only one who seeks her. Savage enemies, seething with dark magic, are hunting her. A daring, determined revolutionary thinks she could save his people. And he looks at her in a way no man has ever looked at her before. Elisa could be everything to those who need her most. If the prophecy is fulfilled. If she finds the power deep within herself. If she doesn't die young. Most of the chosen do.