Maybe it's just me, but the tech level required to read a book online is pretty high.
It's a matter of distribution of e-books being mostly illegal in the western world. When the means of distribution is not effectively suppressed, it is used by almost everybody. See, for example, http://lib.rus.ec/ (Russian e-books' site).
Add to that the power consumption.
You need some light to read a paper book too.
But given the typical price of a paper book $15, you can consume as much as 150 kWh if you don't buy it. That's a lot of energy. It's enough to keep the laptop on for 100 days. I believe much longer with an e-Ink device. And all these money now is wasted on ineffective legacy distribution network. Eventually, these $15 will be consumed as energy or resources, emitted as CO2 and contribute to toxic land-dump in third-world countries, with an effect probably multiple of what you, personally, can do with 150 kWh (see Multiplier effect)
And the paranoia of knowing the ebook I read today can be changed tomorrow to reflect a different view.
That's why you should download it and keep forever. Forget online readers and DRM schemes, that let you see a page, but don't let you have a _copy_. Hard drives are cheap.
Respectful publishers will sign their e-books.