I'm trying to recall what I might have been reading when I was 13. Around that age, I loved mysteries, including Sherlock Holmes and Agatha Christie. I especially remember reading The Westing Game by Ellen Raskin in 5th grade.
I read the Symposium and the Crito when I was 14. Although I found them stimulating, I think they were too difficult for me to follow.
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What age group reads Sophie's World? Is it at the right level for teens? I doubt that I could have maintained interest in such a long book back in those days. (Oh yeah, I guess I still avoid reading books that are more than 300 pages long!)
Are there some Borges short stories that are not too sophisticated? It's been years since I read them, but I loved "The Library of Babel" and "The Circular Path."