I would love it if eBooks supported custom, programmable side-bars, one where the layout designer could add a "map" of the book that was more unique and memorable than the percentage tracker of, e.g. the eInk Kindle.
A significant strength of Kindle is its distraction-free nature. While a progress-bar might be good, I don't think anything more complex than simple line would be suitable. Also its natural place would imho be the bottom edge.
I thought I agreed (and even disabled said progress bar a couple weeks ago). However, the research suggests that there's a counter-productive degree of "distraction free" and that some eReader interfaces pass that threshold.
As a side note: I know that my expectations for a book change depending on my current position. I notice this every time I think I have a large amount of the story left, only to find myself unpleasantly surprised when it ends "abruptly" and that the next half is actually another story bundled with the first in a single file.
The Kindle had one. One of the things I miss from upgrading from a Kindle Keyboard to a 2013 Paperwhite, where I can't figure out any way to get the bottom progressbar back.
Possibly for later novels. The one I have in mind (Planet Oz) keeps the focal characters in close proximity, at least compared to the scale of their travels, so a geographic map could suffice.
