Graceling (Graceling Realm #1) by Kristin Cashore
Publisher: Harcourt
Goodreads Summary: Katsa has been able to kill a man with her bare hands since she was eight - she's a Graceling, one of the rare people in her land born with an extreme skill. As niece of the king, she should be able to live a life of privilege, but Graced as she is with killing, she is forced to work as the king's thug.

When she first meets Prince Po, Graced with combat skills, Katsa has no hint of how her life is about to change.

She never expects to become Po's friend.

She never expects to learn a new truth about her own Grace - or about a terrible secret that lies hidden far away...