Grave Mercy by Robin LaFevers:
After escaping a terrible arranged marriage, Ismae, 17, flees to the convent of St. Mortain. Here, she learns she possesses dangerous powers, bestowed upon her by the God of Death himself. Ismae chooses to stay at the convent and be trained as an assassin. But trouble comes when she receives her first assignment. What happens when Ismae falls in love with the one person she's been sent to kill?
After reading Grave Mercy, guest reviewer Jenna had this to say: "All around a great book! I had a hard time putting it down. The author does a great job describing details and plot."