Start here. This is the definitive history of the massacre. We have a copy of this from the first printing that was passed along in my wife's family as she has family members who were involved in this from the beginning, as well as being tangentially related to Juanita Brooks and her family.
en.wikipedia.org
The Church put huge pressure on Juanita to rewrite the history as it claims Brigham Young and other church leaders were accessories after the fact. But she refused to edit what she had meticulously researched.
Also, if you get a chance to visit the site of the memorial it is worth the trip. They show exactly where the encampment was initially and what path the mormons lead the wagon train folks along while setting up the massacre and exactly how it went down. You can hike along the path they were forced to walk just before the mormons opened fire and up to the point where the local indians helped massacre the women and children. It is straight horrifying. We have been there a few times and walked the path a couple of times. Very sobering. They show the ravines where the human remains were found and a couple of them still have remains that were left in situ out of respect for the dead.
Little of what is portrayed in American Primeval actually happened. It very very loosely pulled from minor and questionable historical accounts. So as a fictional representation to evoke a shock-factor around life at that time in the rugged frontier it is mildly entertaining, but hardly to be viewed as historical in any even remotely serious context.