I've seen this happen enough times to prompt a blog post, but each time I see it it makes me cringe a little.
Recently I saw it like this. Someone was sitting on an aisle seat, reading a book, while someone in the aisle was hovering above them, staring intently at that same book. When the reader turned the page, the hoverer continued reading, still not broken out of her trance. I thought the flip of the page would let the hoverer realize that she was invading the reading space, but she seemed oblivious to how rude she was being. I almost wanted to tap the seated woman and whisper that the hoverer was reading everything over her shoulder and unless she wanted a shared experience she may want to readjust herself.
Reading over someone's shoulder on the metro is not ok, but it's even worse to then make a comment on it. I realize we're in a public place, but generally people aren't reading so that you'll strike up a discussion with them. A lot of times when I'm reading, whether it's the Express or my new favorite novel, I'll angle it so that most people can't read it. It feels like an invasion of privacy when I look up and catch someone else staring at the words on my page. When I'm reading, I'm in my own world, not thinking about the commute or the crowd or how late I'm going to be for my appointment. It's a soothing way to get through the commute, and when my little bit of private space is invaded with an over-the-shoulder reader, my little world collapses and I'm back riding the swaying jerking metro.
Something about over-the-shoulder reading is creepy, I guess for its invasiveness, even though you are in public. But those of you who read anything on the metro - the newspaper, a book, a textbook, a report from work - expect that someone will view what you're doing. It's a tight space and even if others aren't trying to read it, eyes will roam. If there's anything you'd like to keep private (such as a confidential corporate document), don't bring it out to read on the metro.