I'm embarrassed that it took me as long as it did to finish this book, between COVID and moving and having two babies. But it's a good one.
Roberts-Miller breaks with the traditional definitions of what "demagoguery" is, because usually people make the sloppy ethical definition of saying, "demagoguery is what demagogues do" and then define demagogues as these mustache-twirlers who know they are misleading people into dangerous ideas. Actually, most so-called demagogues, including Hitler, were very sincere in their terrible ideas and thought they were leading people to great ideas.
Instead, Roberts-Miller implicates us all into demagoguery--we are all able to be seduced by it and we all sometimes engage in it. Her theory of demagoguery involves three stases: who is the "out group," how bad is it to be out-group, and what should be done to/about them? We engage in demagoguery when we talk about "us" as infallible and "them" as always suspect. Demagoguery shuts down public discourse, because if it's always about in-group and out-group, there is nothing else to say. The out-group's words will always be poisoned at the well for being out-group.
While it's easy to see this in the big stories of demagoguery (Hitler again some easily to mind), there are cases of demagoguery where no one gets killed or disenfranchised. An example Roberts-Miller brings up with her students is PETA's casual bashing of snake owners in a statement about snakes crossing state lines. Snake owners are not going to be discriminated against because of this bit demagoguery, and they probably won't even read the screed. However, it may contribute to an underlining of "us vs them" in the in-group. They feel the battle lines are drawn and they are on the side of right.
What's great about this book is that we aren't immune to demagoguery's appeal, so now we are beholden to think a little more carefully about what we're arguing or agreeing with and why. As Roberts-Miller points out, this doesn't mean radical relativism, but just recognizing that groups aren't monolithic and unchangeable.
I'd still recommend the little book version for casual reading, but this could be a great textbook for a masters-level course on argumentation.