If she likes the Pratchett, she should try Diana Wynne Jones. I recommend her books frequently. I'm always so surprised they are more popular in the US. She has written good kid/tween series of Chrostomancy books (kids), but also Howells Moving Castle (I like it better than the movie), and my favorites The Merlin Conspiracy and Deep Secret which are really more teen than kid.

My teen is currently enjoying the Pratchett books. I tried to get him interested when he was younger without much luck.