I don't even like to change songs on my ipod while i'm grinding live... IMO you should be paying attention to every little thing at the table until you feel somewhat drained, then you take a break, get some food, take a walk. Maybe even cash out and take an hour break until you can get back into your grinding zone. The more you practice this, the longer you can maintain A game sessions. There is no way you can pick up on tells, betting habits, etc while reading a book.
I'm also glad to see people's opinion about sunglasses, hope people think this of me when i have the hood up and glasses on at the table

. When you wear them your opponents aren't aware that you are observing closely!