I love this book, though I've not made it nearly all the way through. For one thing, the authors explained a number of permaculture topics in ways that made them finally click for me.
They also define the Third Principal as Careful Process, which they describe as an ethical catchall. Their definition includes creating a yield, sharing that surplus so that other peoples' needs are met, and limiting consumption and growth.
Most important to me, this is the first place I've seen in permaculture that recognizes the problematic nature of "wilderness" and the erasure of indigenous land management. Because of this they attempt to redefine Zone 5. I'm not sure they succeed in differentiating it from Zone 4 but perhaps that's the lesson to learn, that the whole world is under some amount of management and that some areas are lower use/foraging but none are untouched.
My one criticism is that they reference permaculture design tools that I suspect are original to other authors without giving due credit, particularly ones that I've seen in Aranya's Permaculture Guide, though those tools may be in more common use than I know of, having never done in-person permaculture training.