Trickster's Queen by Tamora Pierce

Aly’s adventure continues. . . . No longer a slave, Alanna’s daughter is now spying as part of an underground rebellion against the colonial rulers of the Copper Isles. The people in the rebellion believe that a prophecy in which a new queen will rise up to take the throne is about to be realized. Aly is busy keeping the potential teenage queen and her younger siblings safe, while also keeping her in the dark about her future. But Aly, who is usually adept at anticipating danger and changes, is in for a few nasty surprises.