Including sub-books into the set-tag

Hello,

I found the "set" tag which can be used to merge separate books into one big book, but in the example, they just wrote three sub-books inside the set-area, which is not what I'm looking for. Instead, I'd need something to reference an external file, so that I have a "set"-file and my separate book files, while the set-file only contains the path and filenames.

Is there any way to achieve this?

Nice greetings
Rock Lobster