If I can add a comment, I suggest that Commons should be included into wikidata net as soon as possible, since it shares with wikidata its feature of being "a unique container" and book metadata are already shared between all wikisource projects and Commons for a central group of works (the "proofread" ones). I suggest too to keep wikidata staff in contact with Jarekt, who works hardly  into Commons and is particularly interested about books and "creators" (i.e. authors).

In my opinion, the fast inclusion of Commons into Wikidata net will solve most issues of wikisource projects too.