By seminal I mean books like Rameau's Treatise on Harmony, Fux's Study of Counterpoint, or a more modern text like Piston's Harmony.
I have scanned translations of treatises by Riepel and Koch. They are tough reads, but important historical sources regarding meter and phrase.
It seems like Cooper & Meyer, The Rhythmic Structure of Music is a key text from modern times.
Are there other seminal works I should read?