> Right now, my girlfriend and I are reading the same physical book. I read it at work over my lunch break and she reads it at night before going to bed. Is this piracy too?

The key concept here is 'excludability'. Only one person (maybe two) can be physically reading it at once. Digital goods, sans DRM, could in theory be shared with pretty much the entire world without depriving anyone of their experience.

http://en.wikipedia.org/wiki/Excludability

This difference between ebooks and 'real' books is the heart of the problem, and why people want to introduce artificial excludability. Actually, I'm fine with that: treat it like a real book in terms of only letting one person at a time have it, but let me freely choose who has it when and for how long.