Poetics

Back in Aristotle's day "poetry" included a much wider sphere than it does today, including drama, comedy, tragedy, and the more obvious epic and lyric poetry - it's a term that literally means "making" in this context. His Poetics (335 BCE) is a philosophical text on aesthetics, dramatic theory and the laws of literature, and at 2400 years old is the first of its kind.

Anyone who studies literature will find tons of huge and unwieldy sounding Greek words that might be confusing, but in Poetics much of them are well-defined. Such terms in this tiny (50 page), yet tightly packed treatise include:

Mimesis or "imitation", "representation"
Catharsis or, variously, "purgation", "purification", "clarification"
Peripeteia or "reversal"
Anagnorisis or "recognition", "identification"
Hamartia or "miscalculation" (in Romanticism = "tragic flaw")
Mythos or "plot"
Ethos or "character"
Dianoia or "thought", "theme"
Lexis or "diction", "speech"
Melos or "melody"
Opsis or "spectacle"

Whether or not you're a fan of Aristotle specifically or philosophy in general, this book is really for anyone who likes to read. Its tenets are universal and totally applicable to any study of today's literature as well.