Read all of Garth Nix' books about the Old Kingdom and Ancelstierre.

I have listed them in chronological order according to the in-universe timeline, but you can read them in any order to complete the challenge.

Bonus 'books' are short story collections. If you're hungry for more Old Kingdom content, the WayBack machine from the Internet Archive has pages from the Old Kingdom website archived which have a  few extra short stories and similar tidbits.