Erm, I guess I'll have to push the boat out and say move away from sci-fi and fantasy, they really aren't as open to interpretation as what I originally had in mind, stuff like
Of Mice And Men-Sooooooooo much stuff to write about this book, is it too short though?
Anything by Dickens-Yeah, you're pretty much set if any part of the assignment asks for 'historical relevancy' or some such.
Ideally you want a book that is relatively easy to interpret the key themes of, and you'll probably get kudos if they are still relevant today, English teachers love that.
If you think you want to do Pratchett, I'm not going to stop you, but all you'll end up is flowering up the fact that his books are simple parody with a small moral usually involved.
PM me if you have more interest.