Having self-studied harmony from excellent modern textbooks (e.g., Laitz, Kostka-Payne), I found myself unable to really comprehend what was going on harmonically in a piece of, say, Wagner. There were, seemingly, too many (random?) modulations, chromatic harmonies, e.t.c., that did not make sense as a practical procedure one would follow in composing.
Schoenberg's book provides a unifying theory on these "romantic harmonic issues". In his theory, the chords/harmonies that correspond to a single key are not confined to the so-called "diatonic" ones, but many more, of almost equal importance, are included, justified as "substitutes", "transformations", e.t.c. These, along with the concept of regions (which is what modern textbooks refer to as "tonicization") provide a large harmonic "space" out of which the composer can draw, remaining in a single key.
Suggestions (rules) concerning voice-leading and resolution of these "extra" harmonies are provided, with many examples and analyses from the literature (ranging from Bach to Strauss).
Clearly, the level of the book is advanced. You should be in a position to analyze a classical-period piece before attempting to read it. Also note, that the organization is not perfect, e.g., there are places where a concept is used in passing before it has been presented/explained. This makes a second (/third/fourth...) reading of the book a necessity.
Highly recommended for anyone interested in these topics!
It deals with advanced harmony theory based on traditional theory. This book introduces Region concept of harmony. It is eccential concept for understanding the structure of harmony progression related with modulation. But I was able to find it only from this text, any other books of harmony do not deal with such concept.