Bush Sr was a junior agent at the time, he helped but others above him were the major players. Mae Brussell and Penn Jones Jr. (his 4 book series Forgive My Grief) exposed a lot of what happened in Dallas leading up to and the assassination itself.
Mae's website:
http://maebrussell.com
Penn Jones Jr.'s 4 book series (vol 1)
https://www.amazon.com/Forgive-Grief-Penn-Jr-Jones/dp/0686212495