Book open, head down does not mean 'Please come talk to me, I have nothing better to do than read this dumb book.'
It means, 'I'm trying to read. I want to be engaged in this activity. This is what I want to be doing right now. Please stop talking to me. I'm already doing something.'
It's not that I'm anti-social, it's just that when I'm reading, I'm reading. Reading.