The Unquiet Grave : The FBI and the Struggle for the Soul of Indian Country by Steve Hendricks. The writing of this book is OK, perhaps slightly above average. The content and style reminded me of all the stories that I read about the lynchings, police misconduct, governmental suppression, etc. of the American South in the 50's and 60's. Only this is the Dakota plains and instead of the FBI being the "good guys" investigating the oppressive government the FBI is now one of the bad guys.
None of this will come as a surprise to anyone familiar with COINTELPRO or the oppression of war opponents in the 1960's.
As a history, there are better ones on COINTELPRO. But it does cover one niche of that era.