Zen and the art of motorcycle maintenance. As a scientist, it helped me to realise that the quest for truth is only a tool: I have to choose my own goal. Edit: I also want to mention "Language in Action" by S.I. Hayakawa. I read it too recently to say that it has had the most impact on my life of any book, but it opened my eyes to orders of magnitude more cases of imprecision in our language than I had ever noticed before.
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