Good description of Apple ][ disk controller

I am pretty surprised that I can't find any good technical descriptions of the Apple ][ disk controller. It's not in the DOS manual and, come to think of it, I can't think of any publication Apple came out with that describes it. I wonder why this is? Does anyone have any good suggestions for reference material on this subject? I've got Beneath Apple DOS, which is of course excellent, and the Bag of Tricks manual (which rehashes a lot of what's in BAD), but nothing outside of those really describes the disk controller circuitry at an even lower level. I want to know what's going on in the PROMs, and why the certain rules of encoding data are why they are (i.e. first bit of every disk byte must be set, no more than one set of consecutive zeros in any byte, etc.) --