Castle by Garth Nix
Tal and Milla have been sent to the castle to find a Sunstone for the Icecarl warriors. Tal plans on getting two, one for the Icecarls and one for his mother, who is deathly ill. No one has reached the castle from the Ice, though, and the path is now falling apart. Will Tal and Milla find the Sunstone and will Tal find one for his mother? What will happen if Tal is caught with an Icecarl?