An obvious technical one: The Structure and Interpretation of Computer Programs. That might have saved me a lot of time to read in early adolescence. (Or maybe it was better to have to figure things out in my own half-assed way. Time travel is tricky.) But something like Francis Bacon’s Essays might be more important. Seeing that lively and methodical a mind right at the horizon of easy readability would have been a powerful clue about how people think and how far afield it’s useful to look for interesting things.
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