Lord of the Flies, The Giver, those dystopian books are not too tough in terms of vocab, but they may cause emotional turmoil for an especially sensitive kid. My son read The Giver this year and I think he was ready for it in a way he wasn't a year ago. His reading ability hasn't changed dramatically, but he has grown more mature and I think he appreciated the message much more.
My dd is less sensitive than my son this way, but I don't think I will prompt her to read these books until she is nine or ten.
