You can do this with the
Action Chains plugin. Attached is a chain that generates and executes a search for series with only 1 book. Add a keyboard shortcut and it is easy to execute.
Another way to do it would be to use an action chain that:
- finds the number of books in any series
- writes the series book count to a custom column in each book
- does a search for the custom column == 1
Something like this might be needed if the search string generated by the attached action is so long that calibre chokes on it. I don't know what that length would be. I suspect it is very large.