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MarjaE- have you considered using Evernote or Onenote, in my opinion they are far more suited to organising resources, notes etc than calibre is, or will ever be.
I use both of them for similar purposes, for which you're using calibre - to organise information. To me calibre is a fancy file manager, albeit a very good one, but its not an information organiser.
I use Evernote as a 'bottom drawer/garden shed' in which to put stuff. If I want to use a 'thing' in one or more projects I normally move it to Onenote. I find it easier to impose structure (hierarchical and/or network) on information using Onenote. Both of them 'publish' links to their Notes, which enables me to link to an Evernote Note from within a Onenote Note and vice-versa. Example: Evernote embeds PDF's better than OneNote, so I keep the PDF's in Evernote Notes and paste a link thereto into the relevant Onenote Note(s)
They both run on Mac and they're both free, unless you want a lot of on line storage. FWIW the Mac version of Evernote tends to be 'ahead' of the Windows version, and not unsurprisingly it's the reverse for Onenote.
I guess what I'm suggesting is that you might want to consider taking calibre out of the loop.
BR