Graceling by Kristin Cashore

Katsa is a Gracelign -- someone gifted with an amazing gift that is signaled by her two different colored eyes.  She has what is considered the most dangerous of the graces -- the gift of killing.  Her uncle, King Randa, uses her as his enforcer and bully to keep his subjects in line.  Until the day she refuses to harm someone.  Now she's left everything she knows to work with Po, a neighboring country's prince, who is searching for why his grandfather was kidnapped.

This is the first book in the Graceling Realms series.  It is a wonderful fantasy novel featuring one of the strongest female characters written for teens.  Katsa is sure of her abilities, knows her mind, and isn't afraid to be stronger than everybody else.  The villain of the book is truly evil in a way that seems completely invincible.  Yet the ending is realistic and completely believable.  This is great series for both teen girls and boys as well as adult readers of fantasy.