The Red Fox clan / John Flanagan.

When you're a Ranger who happens to be royalty, stopping treason isn't just part of the job - it's personal. Most of the time, Maddie is an apprentice Ranger, learning archery, knife throwing, unseen movement and combat skills under the guidance of the legendary Will Treaty. For one month each year, though, she must return home and resume her identity as Princess Madelyn. Forced to keep her Ranger training a secret, Maddie finds castle life dull. But when she discovers clues to a devastating plot against the crown, she realises her family - and the peace they've worked so hard to achieve - could be in terrible danger. To save everyone at Castle Araluen, and her father's forces who are trapped by an enemy army in the north, Maddie must find out the truth. Who is the man in the red fox mask?