You have to go back and read actual documents. I believe the most honest takes can be found in government documents on line...at the Library of Congress. When a modern author on such subjects writes a book...he actually starts with the last line of the book....and includes his prejudices.
Try the library of Congress. It's very user friendly.
While not being an expert on Jesse James nor William Bonney, I have read the most accurate two of many books regarding Wyatt Earp and his history. They are:
Wyatt Earp: The Life Behind the Legend (Tefertiller).
Inventing Inventing Wyatt Earp: His Life and Many Legends (Barra).
With few discrepancies and opinion, they both sustain the Earps as the good guys - contrary to the pro-Clanton (liberal) take and many books trying to prove the Earps as the aggressors. Yes, there were political factions even in a mining town. Both books have footnotes marked for reference which includes actual court documents and deposed witnesses.
Yes, I read documents. Wish the USSC liberal court justices did so with the Constitution and the Amendments.