Chaim, I agree that it would take some work to make it easier to understand what's going on. Rob and I were discussing about ways of "directly manipulating" the zoom level to make it clearer. But as a prototype done in a morning, I'd say this is a success :)
Rob, I wonder what this would look like with an annotated pdf/library:
The yellow could be what you've read (or at least looked at) so far and the red could be sections you've marked/annotated as you read.
The red sections are different opacities because it occurred to me that not all highlight/annotations are equal! Why not have shades of reaction?
In fact, the yellow "what you've read so far" sections could become more opaque the more time you look at specific passages. This would create a sort of heatmap (ugh) of where you've spent your time. And of course this might be extended to everyone that's ever read a work so you could see how all readers have interacted with it.