@speakingtohe: clicking on a partition category (a "1 - 100" list) does not search for anything but instead clears the search, which is why you see all authors. This is by design, because there isn't any single search that can find the items in a partition. I could argue that instead of clearing the search it should do nothing at all, but that wouldn't help you.
There is no search expression that can find books with authors attached to only one book. Searches look at the information in a given book, not information cross-library. The tag browser knows this information cross library, but it doesn't export it in any useful way.
A plugin could be written to produce a list of books that meet your criteria (authors of the book are not attached to any other book). I am not aware of any existing plugin that can produce this list.