That's just the UX of most ebook reader application, it's not a limitation of the format itself. The conceptual model of the most popular epub readers are built upon the concept of a personal library, but that's just because, as you said: "most existing epubs are books". There's nothing stopping anyone from creating an epub file that's not a transcription of a book.

Calibre's e-book viewer should cover your use-case just fine, and in KDE is quite easy to set that viewer as the default application for epub files.