Camelot rising (Kiersten White)

Sent by a banished Merlin to protect King Arthur, a sixteen-year-old impersonating the deceased Guinevere struggles to fit in at Camelot where the magic she practices is banished. Princess Guinevere has come to Camelot to wed a stranger: the charismatic King Arthur. With magic clawing at the kingdom's borders, the great wizard Merlin conjured a solution-- send in Guinevere to be Arthur's wife... and his protector from those who want to see the young...

"The third and final book in the Camelot Rising trilogy finds Guinevere facing off against those she loves most, but can she find the courage to make the ultimate sacrifice?"-- While journeying north toward the Dark Queen, Guinevere falls into the hands of her enemies. Behind her are Lancelot, trapped on the other side of the magical barrier they created to protect Camelot, and Arthur, who has been led away from his kingdom, chasing after false promises....