It's one thing to share some interesting facts and point people to the book. No-one's going to object to that! But I think it's a bit different if someone spends an entire lunch "explaining" the contents of a book they've just read.
It's not a matter of arrogance, pseudo-intellectualism or anything else. It's a matter of wanting to share interesting things one knows.
The phrase "I explained about longitude" paints a very different picture in my head to "I told my friends about this great book about longitude", but maybe that's just me.