What recs immediately lead you to believe someone is credible? Or maybe caught up in the latest fad? Ok, what are the most telling titles? “If someone likes this than this”

Maybe not your favorite all time, but books that reveal something about someone. Mostly looking for good books, but I find I get the most conversation from the ones I think are bad (and thats good- i love talking about books). It seems that the most popular bad books reveal so much about what the average person thinks (or doesn't think about).