Bach is my basic. It’s always back to Bach, the grand master. As a teenager, I played Bach’s Two-Part Inventions, then the Preludes and Fugues (from the Well-Tempered Clavier Books I and II), and then I discovered the French and English Suites, and I played a lot of both.
Bach Well-Tempered Clavier
The Music Animation Machine helps pianists visualize the counterpoint in Bach with videos showing each of the voices.
I think the best definition of polyphony that I ever heard was from Arthur Jacobs, who defined it as “the simultaneous combination of two or more melodies to make musical sense.”
The intelligent listener—not to mention the intelligent student of adult piano lessons—”must hear the melodies, the rhythms, the harmonies, the tone colors in a more conscious fashion. But above all, [s]he must, in order to follow the line of the composer’s thought, know something of the principles of musical form.”