I used Aldwell-Schachter, Voice Leading and Harmony. Part VI is relevant to the composers I mentioned but didn't really explain many of the things I was wondering about. Sure, I can name the chords (most of the time), but on a compositional level there's much more going on.
It also breaks sharply with the old orthodoxy where authors used to talk about everything as key modulations and then struggle to explain things which were really related to the original key area but seem very remote in the new “modulated” key. What Schoenberg points out is that music actually very seldom actually changes key. In fact almost the only time this happens is in the classic “Whitney Houston” where a pop tune just goes up a tone for the last chorus and never goes back. Music in the classical tradition almost always ends up in the same key that it starts in even if it visits some remote areas along the way and therefore as Schoenberg argues it makes sense to understand key areas that are temporary in relation to the original tonality. It also means you end up with a harmonic theory that works perfectly to explain jazz standards and pop music.