The Fall by Garth Nix
Tal's father is missing-assumed to be dead-and his mother is dying, and in order to heal her, Tal must find a larger, more powerful Sunstone for his family. But to get a Sunstone, he must climb up the outside of the castle tower, through the veil into the sunlight. Tal reaches the top . . . only to fall off the tower into a strange world of ice.
The people living there call themselves Icecarls and they too use Sunstones for light, but theirs are the ones that have fallen from the castle. The Sunstone belonging to the clan that Tal meets is old and fading. They send Tal and Milla, one of their own, to get a new one. Will they find a new, strong Sunstone in time without getting caught?