Further adventures in GoogleBooks metadata

As you may have noticed, I'm reading a triple-decker.  (Volume II will be posted shortly.)  Google, which tends to be inconsistent about how it handles multivolume works, has each volume uploaded separately.  While calling up my library, I took a look at the subject categories, and was brought up rather short:

Volume I: Fiction.  (Why yes, yes it is!)

Volume II: History.  (Well, it is a historical novel, but...wait, didn't you just say it was fiction?)

Volume III: Inquisition.  (True, but surely you don't want to class it with scholarly studies of the Inquisition.)

Using all three tags to describe all three volumes might have been appropriate.  Putting each volume into a separate category, however, is surely somewhat...odd?