The Kiss of Deception by Mary Pearson
Lia has no desire to be the first daughter of her royal family. Since she can't change the order of her birth, she has to change her identity. So, she runs away with her lady maid on the day of her marriage.
In pursuit of the maiden on the run are the prince who she left behind at the altar and another young man who has been sent to make sure she never ascends the throne. The only thing is neither Lia nor the reader is sure which one is which and the wrong decision could mean her life.
This is high fantasy with a killer love triangle. It's perfect for fans of Cinda Williams Chima or Melina Marchetta's The Lumatere Chronicles series. Plus, this is the first book in a series, so if you love fantasy set in Medieval worlds, or quests, you can get in enter this story early!