Just a note to draw your attention to MDL-57457, which allows Book chapters to be tagged independently to the Book activity.
Anyone interested can see the WIP version of this code at https://github.com/andrewhancox/moodle/tree/tagging. It's currently feature complete and undergoing testing.
To be clear, this is not fully tested code and there are known issues so please do not test this on a site you care about. There are a couple of known bugs that have already been discovered which will get fixed in the next few days.