I know you griped about this in a different thread, but we won't be doing that, sorry. You can uniquely identify an ebook and its version by using dc:identifier in combination with dcterms:modified in the metadata file. If you desperately need a filesystem-safe string then concatenate those two and sha it.

Sha's aren't human-readable, aren't concise. It's a shit way of doing it.

Not that it matters, most of your catalog is done better by others, I'll just go with them. Why would I want books from people who have an aversion for doing things right?