Almost want to go easy on this because it's targeted at a younger demographic. However, there are better books than Sabriel for that target audience which also have some appeal to older readers. I'm fairly sure that at no younger age would I have enjoyed such lifeless characters. I could handle stereotypical, two-dimensional caricatures, but the protagonist, her cohorts, and those she engaged with barely met the criteria for that description, with one exception.
A young woman is on a quest to find her dad, who she feared is dead. I say feared, but it wasn't conveyed convincingly enough to justify the usage of the word. The lack of imminent peril didn't help, nor did the little time spent establishing a relationship between the two. This is especially true when it's established fairly early on how absentee the father is. I don't know why she cared. I certainly didn't.
There are things to appreciate in this story, like the magic system. But it was like watching someone play Mario Kart and seeing them constantly going off course. Nix may have had fun, but I just wanted someone to play 'properly'. It seems that while the book has some interesting elements, the overall execution left much to be desired.