A Wrinkle In Time, by Madeleine L'Engle I probably read it in fourth or fifth grade, not too many years after it was published. It was definitely the first science fiction novel I ever read. It wasn't until many years later that I saw how foundational to my worldview it was that my first SF book had a female protagonist. I was going to give a few other examples of truly life-changing works (LOTR, Dune, Neuromancer), but I'm not sure how I would have found them if A Wrinkle In Time hadn't opened by pre-adolescent eyes.
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