Grave Mercy by Robin LaFevers has it all: a strong plot, well developed characters, political intrigue, nuns as assassins, and a unique take on religion.
Ismae has always suffered. Given away in an arranged marriage to an abusive man, she barely escapes with her life. Seeking help from the herbwitch, she is smuggled to a secluded convent, one which serves the god of death. There she finds that not all nuns are soft-spoken servants. The Daughters of Mortain are skilled assassins, serving their lord. Ismae, herself, has been called by Mortain and excels at her new profession.
Along with learning about poisons and weapons, the young apprentices also learn about how to use their womanly ways in the service of death. While it seems stereotypicla that every book must focus on the sexuality of teenage girls, based on the tenor of the rest of the book, I can only assume LaFevers was making a statement through this.
While the book drags through parts, the overall story is darkly compelling.
Disclaimer: A copy of this book was provided by the publisher.