IF YOU WANT TO KNOW THE DEFINITIVE JAMES GANG HISTORY...
Try Ted P. Yeatman's definitive nonfiction history, "FRANK AND JESSE JAMES: THE STORY BEHIND THE LEGEND". You can get them cheaply, i.e., used from Amazon.
I knew this guy in college and grad school; he lives eats and sleeps, THE JAMES GANG X 35 years. I get nothing from this and he will not either if you pick up a used copy. Ted and I went our seperate ways years ago but I still respect his award winning definitive works and historical 'beautiful mind.'