Modern Bibliography: 'ser' vs. 'estar' in Old Spanish
I'd like to know if you can recommend me some new books or articles about the origin of the difference in verbs ''ser'' and ''estar'' in Spanish. I know that the difference is present also in Portuguese (and ''gallego''), so I think that maybe the roots of this is in a big branch that includes the ''galaico-portugués'' and the ''leonés'' (which I've read is the basis of Spanish - ''castellano''-).
According to Ricardo Carballo Calero, Portuguese and Spanish were originated as dialects of ''gallego'' and ''leonés'' respectively, which, in time, were originated from a ''protorromance galaico''. If his hypothesis were true, does it mean that the beginning of our structure ser-estar started in this ''protorromance galaico''?
Thank you for your help and sorry for the Spanish words I haven't translated into English (I didn't know how to do it).
(Transferred from old LINGUIST List Ask-a-Linguist site)