Have you read the Wizard of Oz series? There is more than just that one book the movie is based on. I have it on my TBR list and am looking forward to exploring what appears to be quite a rich fantasy world. Other YA books in the e-library include the Narnia series
and the Anne of Green Gables series.
If you are exploring Pratchett's YA don't forget 'Only You Can Save Mankind' and the rest of the Johnny Maxwell books
