A recent question from Tsundoku about the Bhagavad Geeta led me to think about how we tag works whose component parts themselves can be regarded as independent works. The Bhagavad Geeta is a small part of a much longer work, the Mahabharata. It is easily the best known part of that epic, as in practical terms it serves as the authoritative scripture for contemporary Hinduism; for example, Hindu witnesses in court are sworn in on the Geeta. So it perhaps deserves its own bhagavad-gita tag rather than simply being subsumed under a mahabharata tag.
Similar considerations arise with many other works. The obvious example is the Bible. Would a question about the Book of Job necessitate a book-of-job tag? Would we need tags for, say, old-testament or hebrew-bible? And would those be synonyms? If so, which tag would be canonical? If not, how do we decide between the two for any particular question—based on the asker's perspective? Apply both if it's a neutral perspective that isn't specifically Jewish or Christian? Or perhaps just have tags for the specific books, including Apocrypha, plus a larger bible-as-literature tag applicable to all questions about canonical or deuterocanonical Biblical books? Or just the larger tag and not tags for the specific component works?
It's not just religious works that have this issue, of course. J. M. Coetzee's Dusklands comprises two novellas, The Vietnam Project and The Narrative of Jacobus Coetzee. These novellas have no characters, situations, or specific settings in common, but share themes. Each can be discussed separately or as part of a larger thematic whole. Should we have three separate tags for Dusklands and each of the two novellas? Or should we just have a single dusklands for all of them? If we have three separate tags, should a question that's only about, say, The Vietnam Project be tagged with dusklands as well? Because even if the question doesn't mention The Narrative of Jacobus Coetzee, an answer might.
I can see arguments on both sides of this question and am hoping to spur a discussion here.