Wikisource may aim towards being a repository of sources for
Wikipedia. But IMHO it is not its only goal. In French Wikisource, we
proofread some novels and short stories... and we would like them to
be available in e-pub and pdf: now many people read on e-readers and
we must be here to give them free texts to read. Don't you think so ?

I think that Wikisource should be a repository for public domain knowledge and literature.
As Wikipedia is a sum of many specialized encyclopedias, probably Wikisource is (should be)
the sum of specialized libraries... We work with public domain texts, and that includes documents and grey literature, maybe from public sector and legal literature.

Anyway, it would be very important to have an automatic tool for exporting in ePub or pdf,
but IMHO the community is really small for that.
Probably, developers of Wikipedia, PediaPress and Source (ThomasV?) would be able to work together to create a tool that needs to be as automatic as possible.
It is an important goal, the my only problem is that I can't really help with that,
I'm not a developer and I don't know how to contribute to such a technical issue.

Of course, in the Italian community we're taliking a lot about this (also within Wikimedia Italy),
but I do think that we lack people and competences to do something real and significative.

If there's someone out there who's working on this,
we are ready to test ;-)