He writes about the latter :-
Knife Edge is the sequel to the novel Noughts and Crosses. It is a novel about an environment where two races are pretty much at war with each other. It is a futuristic view where black people (Crosses) are the upper class and white people (Noughts) are the lower class. It shows how the two races are distinguished from one another and how they relate to each other. Knife Edge is about two people from different backgrounds who live their lives in a hostile environment.
Philip liked Lirael best of the three, and writes : I liked this book the most out of the three as it held my interest the most and I read it the quickest.
Read the rest of his report here.
Malorie Blackman's website is here, and Garth Nix's here.