There is a fine novel by Loren Estleman which puts flesh on the bones of Billy Gashade and his relationship with the James brothers. Complete fiction but entertaining nonetheless. https://www.amazon.co.uk/Billy-Gashade-American-Loren-Estleman-ebook/dp/B003P9XJTC/ref=sr_1_1?keywords=billy+gashade&qid=1561930601&s=books&sr=1-1-catcorr