From what I know it is only possible to add a playlist in the source list, and
not an extra library.

So I would like to suggest that it should be possible to add an extra library.

The reason is, that I have many full albums, and these are of course in the
library. In addition do I have many songs of which I only one or two songs from
the same artist.

Mixing the list of full albums' artists and list of "single" artists, adds up in
my case to a great deal. As my "singles" out numbers my full albums, I often
only see the artist names of which I only have one or two songs from.

So if it was possible to have one library for full albums and one library for
mixed "singles" the artist lists would be much simpler and more useful.