I've never bought an eBook, but I've used Gutenberg a good few handfuls of times. And I read tons of scientific work in PDF format, and try to avoid printing them -- I like to transfer the PDFs to my Axim and use it to read them while reclining. 🙂 Now if only I could get someone to fan me and feed me grapes....
If you're having trouble with readability on your Mac, try some things like changing colors. perhaps try switching using the invert colors user accessibility feature from the System Preferences. That might help. Everyone is different -- sometimes I space cadet while reading things on screen, but usually it works very well for me.