I want to know how to express every thought and feeling and concept imaginable via the interactions between layered, sequenced frequencies. I can already improvise and compose intuitively decently but not anything longer than 8 bars max without it sounding disjointed. I want a deep, fundamental understanding of music theory. I mainly seek to accomplish this by reading tried and true books on the subject, ideally ones from before the 20th century that the great romantic composers would have studied, or contain the ideas that they would have relied upon.

What are some highly reliable books that can help me with what I'm trying to achieve? Who are some of the greatest music theorists whose work paved the way for the musical creative eruption of the 19th century?