>But you would still find Harry Potter if you were looking in the "Fandoms by Source Text" category, right, because there would be the "Harry Potter" category in there?

Correct!

>Where would the other book fandoms fit in "Fandom by Source Community" terms?

I know there's a Science Fiction fandom category, but there might not be an umbrella term for all these fandoms, I don't know. Not every Fandom by Source Text category needs a "Fandom by Source Community" category.