(probably Magnus' mail was not directed to the Wikisource list, I didn't receive it).
The problem is that books =/= texts, and we have "many to many" relationships at different levels.
> 1. A Wikidata (Qx) item is an edition of another Wikidata item (Qy)
> 2. Qx (the edition) has an associated (e.g. English) Wikisource page
Wikisource has at least 2 pages for books: ns0 and Index.
They are often not the same thing.
For example, I can have an Index page for a poetry book, and have single pages in Wikisource ns0 for each poem.
At the same time, I can have 3-4 index pages for a very big book, which has a single ns0 page.
Every index is a volume.
Moreover, I can have both at the same times: multiple Index pages (volumes) for books that contain different works (think as a 3 volume edition of Shakespeare tragedies).
Finally, the can be a disambiguation like page to in a "work" namespace (this one is easy).
It is solvable, but it is complex.
3. Qx and/or Qy have an associated (English) Wikipedia page
I think that one of the issues here is the simple fact that the matter is complex, and even if you find a solution to the "theory", it's not easy to comply. Right now in Wikidata there is confusion, around books, and you don't really know what to do with them.
Moreover, it is not easy at all to create a new book, make the right relationships with the author and other work/edition of the same book. For example, as far as I know, you can't copy an item, and this alone is a burden to the user who has to start everytime from scratch a new item.
I myself give it up most times than not....
Aubrey