The world he lives in is always dark, there is a veil which keeps it dark, this is important to uphold the rule of magic.
When any of the "Chosen" turn thirteen, after, on the Day of Ascenation where they all enter the spiritual world (Aenir) to get a Spirit Shadow. When the Chosen are under the age of thirteen, they are given a Shadow Guard, which is similar to a Spirit Shadow but less important. When his father has disappeared on a mission for the empress, he has the families primary sunstone.
All of the Chosen have at least one sunstone, they can be used for many things: Healing, Music, Heat etc. But a primary sunstone is essential to enter Aenir, if you don't have one you join the Underfolk who work as servants for the Chosen. Desperate to get a sunstone, he tries to still a primary sunstone from the Seventh Tower. He falls back through "The Veil" and falls to a place he is not familiar with. He begins a quest with a "mad-girl" back to the castle.
I think this is a great science fiction book. It is about a whole other world, the author (Garth Nix) did a great job of it and I look forward to reading his other books.