Fall (The Seventh Tower)

Fall (The Seventh Tower) by Nix, Garth. mass_market edition. ISBN: 9780439544153.

First of a thrilling fantasy adventure series set on the Dark World, where society is ranked according to its colour clan and the most precious commodity is light. In all the world there is only one place that ever sees the sun. A seven-towered castle built upon a mountain high above the desolate ice lands below. Tal is getting ready for the Day of Ascension -- a day when all the 13-year-old Chosen from the Castle of Seven Towers enter the spirit world of Aenir. Then his father disappears with the familys Primary Sunstone. Without it, Tal cannot enter Aenir and bind himself to a Spiritshadow -- a guardian being, both protector and friend. Tal tries to steal a Sunstone, but during his act of thievery is thrown off the Castle of Seven Towers by a powerful Spiritshadow Keeper. He falls down to the iceworld below, where he is captured by Icecarls. To save his life, he must team up with Milla -- a Shield-Maiden-in-training -- and offer his aid to the nomads. Which complicates his task immeasurably!