^I was going to read 'Guns Germs and Steel'... might have to do that when I find it in a library.
I have no idea how 'Sabriel' and the others haven't gathered more attention... some might find it too "young" for them but it's still quite a kick ass mature story and severely underrated. First read it when I was about 10 (advanced reader) and am hoping Nix writes more about the Old Kingdom/Ancelstierre.
...Actually. You might enjoy 'Across the Wall: A Tale of the Abhorsen and Other Stories
'. There's a short story where Nicholas Sayre tries to cross the Wall again. 'Hope Chest' is also pretty epic.