I’m interested in doing some readings in the classical grammaraians. I’ve been intrigued by the idea or the Orthe Ptsosis/casus rectus vs. the oblique or plagal forms, ever since I first encountered that disticntion in the Humez book on Latin. I’d appreciate references and advice from people familiar with this literature.
From when I was more interested in ancient linguistics, I found the following two books useful:
History of Linguistics II. Classical and Medieval Linguistics - Lepschy (specifically the article by Matthews)
The Byzantine Grammarians - Robins
This should be a good place to start to find information.
About actual grammatical works, I know that Dionysius Thrax’s work is available online and I believe it was the basic grammatical work for Greek for a long time. The ὀ?θή πτῶσις terminology already occurs in it.
I dimly remember reading that originally the nominative was not even considered a πτῶσις at first, and that the other cases were seen as modifications (literally “fallings”) of the nominative. So I guess the nominative was seen as the “upright” form (i.e. not falling) or the “true” form, depending on the various meanings of ὀ?θή.