"Ishmael" by Daniel Quinn (as well as "My Ishmael") changed how I view the world and how I characterize contemporary issues. The Takers/Leavers dichotomy, while a tad simplistic, was jaw-dropping at the time. "Zen and the Art of Motorcycle Maintenance" had a similar effect, though with the Classicist/Romanticist split more than any other part of the book. Also: "Itty-bitty rules for itty-bitty people" made quite a few things make sense.
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