Absolutely, we played with the progress indicator for exactly the reason you mention -- how far along am I? But when you have branches that are asymmetrical in length then it's not straightforward to compute. With your book metaphor, instead of a linear book think about a choose your own adventure book and how hard that was to gauge how much of the book you'd read.
Totally makes sense! Perhaps there would be a way for authors to number their own pages (or parts) in some sort of customized way. It definitely wouldn't work for every story, but I could see people structuring by short "checkpoints" or something (like a save point in a video game) out of a total. But yes--definitely a complicated issue.