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petzi - As previously mentioned I use similar 'tagging' strategies to those used by Joanna. However, my experience with amalgamating physical libraries is the converse of Joanna's - it drove me nuts

That's not to say a single library approach wont work for you.
I had three physical 'permanent' libraries - _Main, _Media and _Journals, each of which had:
- 3 common custom columns, and up to 11 library specific custom columns,
- 4 - 8 virtual libraries (17 in all) with little commonality in their search expressions,
- 3 - 5 views (13 in all) with no commonality in column selection, left2right column order, and column sort sequence.
When I amalgamated them I found it necessary to:
- introduce 3 additional VLs 'All Main', 'All Media' and 'All Journals',
- partition the Virtual Library and View names with prefixes - '[B]' for _Main, '[M]' for _Media and '[J]' for _Journals
I persevered with a single amalgamated library for about a month, but I found it too cumbersome. So I went back to the same three physical libraries. If I could have had three
Super Libraries containing the requisite
Virtual Libraries and
View Sets it might have worked out
Another factor is that I have a some 'subsidiary' libraries (_Intake, _Test and _Workshop), which I would never combine with one another, or with the 'permanent' libraries. And I tend to spend more time in them (using the Library Manager), than I do in the 'permanent' libraries.
- Note 1: I run a server task against each 'permanent' library that I access via three pinned tabs in a dedicated browser. This provides 'read only' access to libraries other than the one I have open in the Library Manager.
- Note 2: I don't use calibre's search facilities a lot, other than via things like Similar Books. However, I make frequent use of external search tools (such as X1 and Windows Search - they're similar to Spotlight) and the Drop Search Results (DSR) PI to create Reading Lists, add 'short term project' tags etc.
I prefer this approach because I can widen the search to include multiple calibre libraries, email, blog and commentary posts etc etc.
At one time I shoehorned search result lists into calibre using the Import List PI, but then DaltonST created the DSR PI, which by comparison is a walk in the park.
I'm not sure if DSR works on OSX.
BR