make styles palette at book level (as well as variables, master pages and other shared book elements)
then, there could be a switch button in paragraph styles palette for documents (as well as other palettes) that makes all styles "inherited" from book "superpalette".
I mean, that I would make ONE set of styles which will be instantly propagated into all book documents, (which have this switch in "on" state).
I know it can be done with book "synchronize" button, but it is not working, when you change styles name in one document. Synchronizing just add this style as a new to synchronized documents, instead of changing its names everywhere.
And, last but not least, there will be only one place to check for variables and styles used in whole book.
Alon Emanuel commented
This thing right there - that is the core reason for why the book feature is plain and simple UNUSABLE.