I am trying to include the Reading List into my workflow. And I would like to see items (across groups) which are not in my reading list. But I can’t find a property to look / search for. Is there a way to do that (besides looking at the reading list and doing a manual delta)?
Might relate to Rule to Property to Detect Item in "Reading List"
As noted in that thread, there is no property indicating an item is in the reading list. You can only query the reading list and make comparisions from that list.
You instead may want to consider using a tag or other marker to indicate a file will be read later. This could be queried more directly, e.g., tags:"to read".
Adding to @BLUEFROG’s comment: You could use a script to get all reading list items and tag them with to read (or whatever). And then you can build smart groups collecting all items with and without that tag.
You have to run the script regularly, though, as the tag will not be automagically added to an item that you add to the reading list (or at least I’m not aware of any way to do that).
Thank you for your comments and ideas. I’ll probably go for a custom metadata field which I already use for other stuff. I’d like to see the property/status in a column.
Is there any reason you’re not using the read/unread status?
That can be queried.
Yes, since — as far as I know — the Unread status is switched off as soon as I select/preview the item and for me, that does not mean that I read the item. I want the status “to read” to be more “sticky”, it should require an action by me. But thanks for the hint, I did not think of it since I don’t use it at all.
Just disable Preferences > General > General > Automatically mark items as read.
Thank you, such a hidden option 
Not quite hidden when it’s directly in the first preference pane
Hidden preferences are much more hidden 