Suggestion for adding books from the Kindle folder (library) to Calibre on the PC (Sorry but I don't speak Apple and don't know if there is a similar solution.)
1) Download and install the "Everything" search tool from
https://www.voidtools.com/downloads/
2) Open Everything (When you first open Everything it quickly scans your hard drives, builds an index DB, and saves it for future use.) and search for "R:\Libraries\Kindle" .azw
The quotes are necessary the "R:\Libraries\Kindle" identifies where Everything is to search and the .azw identifies what strings you will search for. You can save the search strings as a Bookmark. I have a rather large amount of storage (about 55TB) and find this is the MOST used application on my PC. I have saved numerous search strings (Bookmarks) for searches I often need. Once you start using it and reading the help I think that you will find this to be a very useful application.
3) In the search results single-click on the "Date Modified" column header to sort on the date. Now you will see all of the most recently added books.
4) Select the desired books, drag-n-drop the selected books to the Calibre window and the books will be added.