i too have had my thinking heavily influenced by Valerie Vaughan's book Persephone is Transpluto. the book was revolutionary to me in so far as how i interpret astrological house rulerships. and when Eris was discovered i thought for sure this was the planet that ruled Libra. Valerie made such a strong case for Venus ruling Taurus exclusively and the sign Libra having less to do with Venus and more relevance to the myth of Persephone. you could say i really wanted Eris to rule Libra more than anything. just to prove that Venus was not the ruler as popularly believed!

my opinion has changed on this, though. in my opinion the true ruler of Libra has not been discovered....yet. as far as what sign Eris rules, well i don't know if it does rule a sign. there is an astrologer named Lynn Koiner who i highly respect who has written much about Eris and Transpluto(the true ruler of Virgo, not Ceres). take a look at her articles:

Eris

http://www.astralis.it/10_oua.htm

Transpluto

http://www.astralis.it/tp.htm

Lynn makes a good case for Transpluto ruling Virgo and when you test this out in your chart using transits and solar arcs you may certainly see it this way!

Lynn's view on Eris is that it is such a positive influence in the chart that it goes against the naming of the planet or the myth! i personally think they misnamed this planet for the effect of its energies has little to do with the name and the myth. that's been my experience and i've examined the arcs and transits of this planet. they all seem positive!

A lot of this may go against what i originally stated regarding Valerie's book but i think the one message from her book is quite crystal clear: Venus does not make sense as the ruler of Libra and the planet that rules Libra has not been discovered yet!