You could set up a custom column that had values like "Currently reading, finished" and so on, and then set up a virtual library that searches for that information. Then you would just mark any book in your library with that information, and it would show up in the virtual library.
For example, I have a custom column Status (lookup name: "status" type: "Text, with fixed set of permitted values") that I use to mark books that I have already finished, that are in my to-be-read queue, or that need to be fixed to my liking. Then I have a virtual library called READ with a search expression "#status:"=Finished" to filter any books I have already finished.
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