Comment by ziddoap

From https://annas-archive.org/blog/all-isbns.html :

>We started mapping ISBNs two years ago with our scrape of ISBNdb. Since then, we have scraped many more metadata sources, such as Worldcat, Google Books, Goodreads, Libby, and more. A full list can be found on the “Datasets” and “Torrents” pages on Anna’s Archive. We now have by far the largest fully open, easily downloadable collection of book metadata (and thus ISBNs) in the world.

So, it your books would need to be present in one of the databases that Anna's Archive scraped, at the time they scraped it.