what did they say needs to be done differently for a rear disc wheel?
For a rear disc wheel, you run the spokes from the opposite "side" of the hub flange compared to a non-disc wheel (on the disc mount side).
This means that the spokes on the outside of the flange (which apparently do more of the work) are the ones which are stretched by the braking forces and therefore resist said forces better.
Like I said, it was my first wheel, so I'm just repeating what the book said! It's my one slight criticism of the book – even though it's been updated well and covers a lot of very modern components, there are a few places where it could do with being re-written so that the modern stuff is the "normal" way and the more archaic stuff is the optional extras.
e.g. the default wheelbuilding instructions effectively only apply to front, non-disc wheels. Given that probably 80% or more of modern mountain bikes now have disc brakes and that 50% of all wheels are, of course, rear wheels, then these instructions only apply to 10% of all wheels.
Still my fault for being a muppet and not reading right to the end of the chapter though.