From my point of view, an upload form should be focused at Wikidata more
than at Commons, anything else is back-to-front.
If we are talking about a published work that it is published is its own
"notability" and transcends whether it is at Wikisource, Commons, or
Wikipedia, such that it is published makes it Wikidata-able (to coin a
word). We can easily support this statement as copyright alone will prevent
a work from appearing at Wikisource or WikiCommons, and similarly some
published works may not be individually notable for Wikipedia, but may be
so for other reference, thinking here of things that have a DOI.
*Then* comes the issue of which site wishes to utilise the data. So
having Wikidata as the primary entry point to enter "book" data, and then
call it from other places as required seems the logical place to start for
any new work at any of the places.