Ah, ok, after pasting a NON-pdoc .mobi into the docs folder...it does, indeed, show up in books, and doesn't, interestingly enough, show up under the docs tab.
So yep, either Documents or Books folder is fine.
Conversely, of course, this means that any .mobi WITH that tag, in either folder, goes into the Docs tab. You'd think amazon would make this clear, they have to expect people to load their own books, one would think
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