I guess I was seeing a big distinction between telling your friends _about_ this great book you've read, and telling the story yourself, as if you were the expert, as if you, rather than the author, had done the research and formed the narrative. I got the impression that LiamMcCalloway had done the latter, because he said he "explained" to his gf about longitude (and doesn't mention the book at all). If you start to "explain" to people on the basis of having read one book, that's being a know-it-all. Yes, this is an uncharitable reading of LiamMcCalloway's post, but I don't think I quite pulled it out of thin air. LiamMcCalloway, if I've got you wrong and you are in fact an intellectually honest person, I apologize without reservation!
If I don't make any more replies, it's because some people who don't like the tone of my original post are taking the opportunity to downvote everything I say.