Sam Anderson, New York Times Magazine
Oh man, I suspect you're going to be hearing this answer a lot, but: the complete works of Ayn Rand. I discovered them toward the end of high school and walked around for a couple of years giving Howard Roark-like speeches to everyone about “the highest blazing good of selfish free-market epistemology” or something. In retrospect, it seems pretty clear that my Objectivist phase had more to do with the subjective agonies of post-adolescence (insecurity, narcissism) than it did with pure reason. (And you could argue the same thing about Ayn Rand's relationship to it.)
Alexander Chee, author of Edinburgh: A Novel
In high school I read a brisk mix of science fiction, fantasy and gay potboilers (Mass Effect 3 now is perhaps the best way to imagine my brain then). So, the novels of Marion Zimmer Bradley—I still can't look at Mists of Avalon—plus the Darkover novels, which all had some gay potboiler action, now that I think of it. And hello Gordon Merrick, famous for The Lord Won't Mind but I am thinking of The Great Urge Downward.
Yes, that is the title.