One request/suggestion to the author: add wikidata links to the books that have wikidata entries, that way people can cross-reference public domain status for their area, look up the book in other places linked via that (be it wikipedia, gutenberg, etc). I think for people interested in learning more about the books it would be a great way to see more info about them. You could also pull that data into the site directly if you wanted to.
Wikidata modeling for books is quite non trivial, there is a distinction between the "underlying work" and its "editions and translations" that adds complexity for typical use, and isn't even totally clear at times (e.g. is X an "edition/translation" or a new creative work that's merely "based" on a previous one? It depends - different people can answer the question differently! Is manifestation A the "same edition" as manifestation B?). Even Wikisource.org, which is a Wiki project, hasn't managed a comprehensive integration with Wikidata as of yet!
Regardless, the OP is doing some great editorial and curation work, and this is something that many other ebook resources don't have. It's great stuff.
Agreed, it's incomplete and imperfect, but I still think that it would be a good addition. And I also definitely agree that the OP does great work, I bought the full collection just before writing my last comment.
For example: https://www.wikidata.org/wiki/Q768792