Probably the first relatively complete book devoted to tonal harmony was the Traité de l’harmonie réduite à ses principes naturels of Jean-Philippe Rameau in 1722. The essentials have not changed in the hundreds of books devoted to the subject since then, and the music that evolved between Rameau and Wagner has followed the same principles of voice leading and modulations and such. Of course, the ideas of Heinrich Schenker and the evolution of Schenkerian analysis extended our views of what tonal music was all about, but that was almost 100 years ago, and I have not seen much real progress since then. Recent music theorists have, however, found visualizations of the tonal system that are new, and which give us a much clearer view of what Bach and Beethoven and Brahms were really doing, and it is worth summarizing some of these techniques, particularly because they relate to some of the graphs we will be looking at when we talk about Other harmonies (...).
Image: H. Schenker - Analysis of J. S. Bach, Choral: "Ich bin's, ich sollte büssen," St. Matthews Passion, detail.