It builds on the idea of 'bookmarks' and we have a new take on them called 'pagemarks' which is basically a box that spans multiple pages.
This way you can keep track of the parts of the book you've read and jump around.
I like reading multiple textbooks, articles, etc and go back and forth often.
This allows me to easily suspend and resume.
I worked on a proof of concept about 6 months ago and I've been expanding it in my spare time.
You can read about it here:
https://getpolarized.io/incremental-reading.html
Polar is free and Open Source btw.
It blew up on HN a few months ago:
https://news.ycombinator.com/item?id=18219960
I'm about to re-announce as we're just in the process of porting it to the web to make it more accessible to people (mobile, tablets, chrome OS, etc.).
It goes FAR behind this though and supports document management, offline, and supports capturing web pages for offline usage and archival.
Additionally it supports annotation so you can take any page and add highlights, comments and flashcards.