Thanks for comments.
Relationship between wikisource and Commons is very strict, and there's a large 1:1 match between structured wikisource data stored into well-formed templates (used into nsIndex and ns0) and Book template; there's too a 1:1 relationship between nsCreator into Commons and nsAuthor into wikisource.
Djvu and (less used) pdf files are already shared among different wikisource projects, but data stored into information page of files are not shared, so that any project rewrites them and stores them with a variety of formats and contents, introducing redundance and mining deeply coherence.
So, we are going to use Commons metadata - already stored into Book and Creator templates - to share them widely anong all projects that need them. When metadata are uploaded and parsed they can be used to feed local wikisource templates and/or to align data with automated procedures.
Thanks for API suggestion, but the question is: does it violates "same origin" AJAX policy? I can read anything by a bot from any project, but AJAX is great to enhance interactivity and to help user just when user needs data, i.e. in edit mode.
Yes, the solution will be CORS, but I can't wait for future enhancements when data can be accessed and used today, with present software.
Alex brollo