I was just going through my notes and highlights in a book on Kindle -- a frustratingly awkward process, but I won't bore you with an account of this year-old machine's limitations. What struck me was the latent possibilities in an option I never considered: "view popular highlights." Kindles show you what other readers have highlighted, a feature I've always considered an annoyance, like reading a marked-up library book. But now, on the plus side...
Suppose, as an experiment, you took a scholarly book and read only the popular highlights -- the passages other readers had found most significant. I suspect that for many books, my graduate thesis adviser's 80% rule for reading in a foreign language would apply. It's this: if you read a page in a language of which you have run-of-the-mill academic command without any external aids or special effort, you might understand 80%. To get to 90%, you might spend twice as much time, and to 95%, twice as much again.
For many books, reading the popular highlights might give you 80%. Why would you want to? If the book's any good, you wouldn't. In any case, 80% of what? On what scale do you weigh what you missed?
For one reading group, though, I think the experiment would have value -- and probably, visceral interest: authors. How fascinating to see what most readers consider most significant. And you might get material for a great 'short version.' I spend a good deal of my professional life cutting articles down to specified word counts -- say, 1300 words to 1000 or 800, sometimes 4000 to 1200. Generally, if the reduction is less than say 33%, the article is improved.
Suppose you used popular highlights for your 800 page book to create a 25-page or 50-page version. If the original is any good, the short version wouldn't be a substitute. But it might be very useful to people doing copious literature reviews, or to those with casual interest. It might be most useful of all to you -- a compressed sense of what readers take away.
Of course, the idea of an extended precis is as old as the magazine book excerpt -- or rather, adaptation. But one based on dozens or hundreds or thousands of readers' as-they-go decisions might look and feel rather different.