In their books, "Capitalism and Schizophrenia" authors Deleuze and Guattari have a chapter dedicated to the novella.
You can find the books publicly, but if you're just interested in this chapter, I've uploaded it here. (It's really helpful. Have a look.)
Their theory of the novella is "what happened?"
In cases where novellas focus on "mysterious" events, they are distinct from whodunits because "it" is unknown.
My novella follows this principle, at least in part.
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