You should feel free to stop someone on the subway and comment on the book he/she is reading. Mention that you read it. Ask how it's going. Talk about the author's other work. That's neat. We are all one community. That's part of the fun of public transportation. And sharing a common language.
You should not ever say the following when someone is twenty pages from finishing a book.
"Oh no. Don't finish that here. It's really sad. You'll cry in public. Everyone dies at the end."
Fuck you guy on the 1 train in a Crocodile Dundee Hat. Fuck you.