I've asked several questions about fairly obscure short stories. I, for one, don't think that these warrant their own tag, but I'd say that the general rule should be to create tags for most major books and book series, but to tag short stories with the short-stories tag and the corresponding author tag.
There are several things that need to be discussed, including what you might consider to be a major book series, so what are your thoughts?
Why not just leave things to grow organically?
I once heard a very wise SE user describe tagging as an "emergent folksonomy":
I try to look at tagging as a kind of lazy topiary: let them grow however they want until they get in the way of the path, then trim 'em back.
During the private beta, everyone has the ability to create new tags and to suggest tag edits to questions. Let's just allow the tagging system to propagate as it will, and only start making drastic changes if things start to be really problematic (e.g. if a tag like fiction or spoiler starts being used on dozens of questions).
TL;DR: you're overthinking it. Why do we need to bother about deciding precise tagging rules at this point? Just use your common sense.
Evaluate on a case by case basis for authors, books and book series.
Without the ability to look into the future—which I would use for other stuff—we cannot predict what will be a major book series on this site. It doesn't really matter if it's hugely popular and earns billions, if no one asks questions it doesn't merit a tag.
We have five tags per question, so author, book title and series title take up three at the most. Let's give any work that is at least a novella the benefit of doubt that it will be a work that merits its own tag. Just consider The Martian: it started as more or less unnoticed e-book, without a crystal ball there was no way to predict its success; it didn't even have an ISBN at first.
So if the OP thinks the work they are asking about merits a titular tag, so be it.
