After trying out the book workflow for 6 months, designing 3 books with it -
I've concluded that it's a completely broken feature, that makes my work 160% more time consuming.

I'm literally in the process of moving all documents of a current book I'm working on into a single document, because it became so frustrating.

So the main problem (among many others), which has already been posted here and seemingly ignored - is the entire way that styles and parent (master) pages are managed in the book workflow.

My workflow, and I can't imagine it being that different for other designers, is as follows:
I create a book with let say five documents in it, each representing a chapter or whatever. I then proceed to work on each of these documents, and as the design process advances, I create more and more styles - object, paragraph, character etc.

And to make sure everything is synchronized, I make sure that the document I'm currently working on is the style source. Whenever I move on to a different document - I synchronize the book, move to the other document, make it the style source and then I keep on working.

At first I didn't see a flaw in that workflow. But things get very messy very quickly. After working on a book for while, you start to rethink your styles, the logic behind them, the way they're organized and grouped together. And then you start deleting styles, and changing names, and grouping them.
But the next time you synchronize the book, all the changes are ACCUMULATED rather than SYNCHRONIZED. That is, my documents become a dumpster for every style and parent page that I ever used.
(as can be seen in the screenshot attached)

Getting them in order is painstaking, and the fact that parent pages can't be grouped together - just adds to the mess.

There should be a better way to do this. And there is.
Styles should be on the book level. Simple as that.
And a user that feels like he really could use document-level style and parent pages - grouping styles by the document name should suffice.

I can't imagine this being SO difficult for the indesign team for it to not be a viable solution.
InDesign is a software aimed for professionals, and it simply isn't robust enough right now. Many crucial features are missing and have been requested for such a long time - whether they're at the core of book design, or whether they're simply needed for effective editing and designing.

I REALLY hope to see a major update tacking the book workflow, as it's much needed, but sadly unusable.