Who is Nicholas Branch from Libra and what is their importance?

Nicholas Branch is a researcher for the CIA who has been given the task of writing a book about the assassination of John F. Kennedy. Branch has a special, fireproof room in his house that the CIA had built for him where he goes over the many, many documents the CIA has collected on the assassination. Branch finds himself reviewing material that has little or nothing to do with the case except perhaps in someone's wild imagination. With the amount of material collected and given to him, Branch begins to wonder if someone, somewhere has decided to sabotage his efforts in the hope that he will never find the truth.