it's amusing to read about the effect Peter Matthiesen had on some folk at the time, it's safe to say, I think, that he scared the crap out of a few people.....example:

Bill Janklow, the former Republican state attorney general and governor of South Dakota, and David Price, an agent with the FBI, filed libel suits against Viking Press for contents of the book. Janklow also filed a suit against the author Matthiessen. In an unusual action, he filed a suit against three South Dakota booksellers for selling copies of the book. Viking Press filed a countersuit against Janklow, which in part alleged he had interfered with the company's constitutional rights to publish and distribute the book. The lawsuits prevented the book from being published in paperback for eight years

reads to me like someone/someones had something to hide, oh and yes I have read the book, a number of times

Ed T is right, conspiracy theories are decidedly unhelpful in any context.They sometimes make amusing reading but in general are tiresome to the extreme.