This is an excellent successor to Matthiessen's "In the Spirit of Crazy Horse" and breaks ground well beyond that book.
First, despite the most arduous FBI efforts to fight his every FOIA request, to do CIA-level blackouts on what it did release and more, Hendricks has more information at hand. He's got enough to make the quite proper judgement that what the FBI did to the American Indian Movement was unarguably part of the notorious COINTELPRO.
And, it worked far better than against black civil rights group or the antiwar movement. That's because American Indians were and are a smaller demographic, and vis-a-vis black civil rights, AIM was the only "pot to whiz in" for Indians vs. multiple black groups. It's also because of the thuggish, illegally empowered tribal government at the Pine Ridge Reservation, the BIA that was in cahoots with it, and the white "hang em high" judges, state and federal alike.
More than Matthiessen, even, Hendricks details that white thuggishness, while not looking quite as much as Pine Ridge.
More than Matthiessen, Hendricks then looks at the results of all of this. AIM populated with a mix of snitches and agents provacateur, for whom he's got a lot of FBI-related info. A growing paranoia from all of this, that lead to other people being presumed to be snitches. And ultimately, the apparent AIM-condoned assassination of an Anna Mae Aquash.
He also doesn't polish the apple of AIM leaders like Russell Means and Dennis Banks as much as Matthiessen. COINTELPRO-induced paranoia aside, and Hendricks' allowances for their pre-Wounded Knee prison time also aside, Hendricks shows both as relatively unsympathetic figures. That includes how, parallel to other liberal protest movements of the era, other than (obviously) women's liberation, sexism was fairly rampant.
He also, without throwing him under the bus, does less apple-polishing of the central character of Wounded Knee II, Leonard Peltier. He still says that he should be freed, but indicates he has no doubts Peltier fired the coup de grace shots on both FBI agents.