I think the point that In Which A Book Lexicon is Born - #12 by tynanpurdy.com made here can be used to handle this (by pointing to not only progress but to the ref of what’s being reviewed, it - the referenced item - can have all of the identifiers needed to identify it across things - Yelp, etc) to make it more rich.
I also want to suggest folks to move conversation about book reviews to this thread: Handling Reviews for the Book Lexicon and keep this thread about how we’ll define either the generic form/interface of a book lexicon (something each app extends in some faithful way or uses under the community.lexicon.book umbrella).