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)