That is from Venkatesh Rao, retweeted by Ben Southwood, a version of “Questions that are rarely asked.”
I suppose if you haven’t read the Bible or Quran those are easy answers, but let’s say you have.
I’ve only read snippets of Mein Kampf, so that has to stand as a contender. But has the book really influenced and shaped my life? Maybe you can attribute the relevant marginal product to the life of Hitler, with the book being intermediated by Hitler himself. Therefore I am not sure that answer is true to the spirit of the question.
How about a training manual of some kind, which perhaps my early teachers read but I have never seen or even heard of? Might my mother have read Dr. Spock or other parenting books? That would be my best guess.