Yeah I was toying around with that, too… but folks often mess around with metadata in tools like Calibre and Audiobookshelf in ways that wouldn’t have an impact on Storyteller’s sync, but would change their hash. On the other hand, I don’t know how various publishers handle EPUB dc:identifiers and that may not be robust enough, either. We could try doing something like hashing only the contents of spine items (including their file names, since that’s how media overlays refer to content)
I was going to suggest to use the same approach as the old CD tagging systems. Count the number of words in each chapter to create a "book fingerprint".
It's highly likely to be globally unique, and it can also help with the missing forewords/afterwords/bonus content sections.
In addition, you can also add fuzzy matching for the title.
I think that the thing we need to account for (which, number of words per chapter would capture this, I think) is different publications of the same book, which would need different overlays if they have different chapter filepaths, etc.
