Schoenbergs theory of harmony text

I've got a copy of this text and I was wondering how good it is. I've read that its more of a "historical" thing than a teaching method, is this true? I have an intermediate understanding of harmony and I would like to learn more, is this a good book for that purpose?

I've had it for a while, and always enjoy dipping into it randomly. The actual "theory" part of it is certainly useful, and it certainly goes into a lot of detail. But I find it much more interesting when he goes off on a tangent (which he does rather alot!). It's very enlightening from the point of view of aesthetics! Of course, it's best to realise that there's an aweful lot of opinion in that book.

His description of diminished 7ths as "vagrant chords" has always made me chuckle.