If you're talking about Categories in catalogs, use tags like this:
The catalog builder ignores tags wrapped in brackets (assuming you haven't changed the default settings). When you want to download a particular subset of the library for [Son], search for
which will display only the books tagged with [Son]. You can now download that subset to a connected device, and build a catalog with only those titles included. Because the tag is wrapped in brackets, it won't be included as a Genre.