I agree with the people above who suggested using a Virtual Library.
I'd like to add, that you can create a Content Server that is
restricted to the contents of a Virtual Library, and connect to that Content Server with your OPDS-compatible app.
You can also start multiple servers via the CLI:
Code:
calibre-server --port "8080" --with-library "/path/to/library/" --restriction "EPUB-only"
calibre-server --port "8081" --with-library "/path/to/library/" --restriction "MOBI-only"
calibre-server --port "8082" --with-library "/path/to/library/" --restriction "PDF-only"
And create the matching "EPUB-only", "MOBI-only", and "PDF-only" Virtual Libraries.