This book opens with an impressive modesty, a dedication from Schoenberg that says something like:
'I learned this book from my students'.
Schoenberg, as many of you know, was not an academic, someone who theorized, but a composer who taught what he himself had learned throughout his great experience. I never understood his music (I have quite traditional, conformist tastes, let's say), but it didn't take me long to realize that one can still learn a ton from his books.
This treatise deviates quite a bit from other harmony books, and it is also a philosophical treatise on harmony; therefore, sometimes the long digressions Schoenberg makes can cause a bit of distraction. Aside from this, in my opinion, it is a masterpiece, period. I studied it years ago when I decided I was fed up with learning harmony and composition from popular music books, because I couldn't learn anything from them, not even from the best jazz books; sometimes I would tear them up because I understood nothing.
And not because I was stupid.... but because, in my opinion, harmony and composition can only be explained best by classical veterans.
I was tired of vague explanations, or of books where there are a million things, but where none are treated beyond superficiality.
This book was the cure.
It starts from scratch, so it is also suitable for absolute beginners (by beginners I mean beginners of classical harmony, because it is impossible to get anything from this book if you don't know how to read music), up to the chapter that evokes images of a harmonic explorer, titled 'At the Frontiers of Tonality' (at least I believe the Italian version is titled that, I studied the English version).
However, one should not think that this is a book where Schoenberg teaches his atonal technique (I have heard of many who bought this book with this expectation, which in my opinion was wrong). Schoenberg believed it was of vital importance to learn how to use well all the harmonic techniques that had already been created before, rather than jumping into atonal techniques, so to speak.
Unfortunately, I had to abandon the last third of the book, but what I learned has made an enormous difference in my music.
(As a side note, I later bought all his other books, from which I am learning a lot)
Thank you, Mr. Arnold.