For normal eBooks, various software makes books very readable on the Nexus 7. For those formatted like the book (as in the case of me in one case) there might be a benefit to a larger tablet, but not enough to make me carry a 10" device. I bought the Kindle version of my Anatomy and Physiology textbook, and it was fine on my old tablet (Samsung Galaxy 2) and even better on the new Nexus 7.
The text can be a little small in portrait mode, but definitely readable on the Nexus 7, and I am due for new glasses. It can be zoomed in, which is what I have done to read a column at a time in the text. In landscape, where you can swipe up and down to move up and down the page, it is very good at being readable with the whole width of the page at once...
The diagrams are very sharp!
I don't think I would like carrying around a 10" tab, especially since I am a relatively slow reader, and I don't mind zooming in and swiping to move around on the page every couple of minutes for those books that require it. Those are also the books I tend to read more carefully, so it works fine for me to scan around a page to read different portions.