[Eugenie]

First, thank you very much eschwartz. That worked.

Second, I think I am losing my mind (or my memory, or both). All this time I have been wondering what's going on with my 2 plug-ins. Neither has been updated to work with calibre v2.3, which is fine. But why was one still visible in calibre (Manage Sony x50 Reader Book List, which lets me choose which 3 books appear on my Reader 650 home screen), while the 2nd one (which updates the read/unread status of the books on my Reader 650) just uninstalled itself and disappeared?

Well, it turned out there never was a 2nd plug-in. When I opened the configuration directory > plug-ins, I found a .json file for ManageSonyX50BookList. When I viewed it, I saw the following (in part):

{
    "booksList": {
        "books": "selection", 
         "strategy": [
          "oldest"
        ]
...
    "unreadList": {
        "books": {
          "query": "not (#read:yes)"
     }

So the Manage Sony x50 Reader Book List plug-in handled both functions -- it updated my Book List and my Unread List. D'oh, double red face for me.

And eschwartz et al, the calibre behaviour is exactly what you stated.

Thank you! Thank you! You have saved me lots of searching time for a plug-in I never had.

The only thing left to wonder is why I didn't see the Plug-in Updater plug-in within the configuration directory > plug-ins. Is that one considered a "non-third-party" plug-in, so it's stored in a different place?