Review of Poetics (ca. 355 BCE)
Aristotle (writer).
This review refers to the first of the original’s two books. The second book is lost.
A normative literary theory, principally of tragedy.
Because it survived the Dark Ages and contains a lot of good ideas with the bad, this one guy’s personal opinions on the theatre are one of the most influential things ever written. In the category of non-religious, non-fiction texts you can still read, I don’t think there’s any competition.
References here: Whisper of the Heart (1995), A Memory of Light (2013).