I wouldn't recommend Partch's book to a student looking for a starting
text on typical theory/composition. Harry Partch was an amazing and
idiosyncratic composer who designed his own instruments and trained
musicians on them in order to get his music heard at all (shades of Don
Van Vliet), he was pretty far from the mainstream.

I actually wouldn't recommend Partch at all for composition, now that I think of it. I would highly recommend him for understanding the intervals and consonance and dissonance ( i.e. a guide to harmony), which I think is more valuable than all the ABC "theory" in the world. There really is no other way to understand those things but to deal with the frequency ratios and tonalities. Anything else amounts to "you can play this chord before this chord, and a lot of people find that sound interesting, or some people like to play this chord before this chord, or this scale, or this key change". Beginning students have such a hard time understanding the ABC theory precisely because it is arbitrary.

Wendy Carlos has some interesting commentary on Partch's contributions.
She notes particularly that Partch's instruments were perhaps not the
best designs for projecting harmonies based on just and other
intonations (HP's instruments were mostly percussive, with not much
sustain).

I never heard of her observations before, but this is one of the things (note: one of) I can't stand about Partch's music. Why would you devise a system based on exact tuning of natural harmonics and then use nothing but instruments with highly inharmonic timbres to realize it?

-Chuckk