Adult Books you Would Recommend for a 13 Year Old?

I don't know your feelings on science fiction, but I would recommend "Ender's Game". The entire Ender Quartet is amazing. It is probably one of the most thought-provoking series i have ever read. Although it takes place in space, it isn't completely science fiction; I would say that it's an almost-philosophical. It is basically about humans and their journey to discover themselves morally and otherwise. Sorry if this doesn't work out for you but i thought i would suggest it anyway. :)