Someone recommended I read this book, and Im still forming my thoughts on it. But it is definitely dunk-able, and if anyone has read it I'd like to hear their thoughts
Edit: the big picture complaint I have is that they separated movements into the false dichotomy of "violent" or "nonviolent"
A ton of "Nonviolence" books from a somewhat neoliberal perspective can be found over at aeinstein.org (the one Gene Sharp created).
Some books out of there are good for tactics and overviews what existed, but one ought to not take them at face value and remember that their angle is not that of Marxists or class struggle focused people. There is also some critique that the CIA might've financed some of the non violent research to discredit the Marxist groups.
That said, the strategic aspects of campaign building in them are good to keep in mind for some. Also it is good to think about the reaction that will come and that too often is ignored by utopists.
https://www.jacobinmag.com/2019/12/gene-sharp-george-lakey-neoliberal-nonviolence
https://www.jstor.org/stable/423870
https://civilresistance.info/challenge/femcrit
https://www.dissentmagazine.org/article/the-machiavelli-of-nonviolence-gene-sharp-and-the-battle-against-corporate-rule
Many aspects in the writing of Helvey et. al. are more acute than the book OP quotes (even if it is an interesting read). Non violence works - as others in this thread pointed out - because of the specific situations and the threat of violence that might be. Also the type of campaign matters. Not the non violent protests lead to change in South Africa, it was a lot more and especially the militant actions which were combined with non violent ones, yet in talks about that often it is presented as violence free.