Emily Windsnap and the Land of the Midnight Sun: Book 5 by Liz Kessler
Emily and Aaron are sent on a top secret mission by King Neptune. The king has been having nightmares he doesn't understand and he knows only that Emily and Aaron must go to the Land of the Midnight Sun to avoid catastrophe. But when the friends arrive in this icy world of mountains and glaciers, they uncover a mystery more dangerous than they ever imagined.
A magical adventure about the power of friendship.