This is part of the trail excerpts available through the church history website. In this bundle are many useful resources. It includes discourses from Brigham Young, history of rescuers, newspaper articles, and remembrances written by Martin Handcart Company members.
The most useful I have found is an autobiographical sketch written by John William Southwell. He was part of the Haven group, which was later combined into the Martin Handcart group at Florence. He gives insight into why there was a division, as well as describes many of the happenings of this group. AN interesting insight he gives is describing the actions of the young men, the day they were caught in a serious thunderstorm without their tents. The eventually got them up, and had shelter over their heads. However the were standing in two or three inches of mud, as they did not have floors. The young men took knives and hatchets and chopped willows from a grove, and with these they were able to construct a floor of a fashion for the tents, and get they Saints out of the mud.
Brigham Young describes how his thoughts were with the Saints out on the plains. He described how he couldn't get them off of his mind. There is a report from Joseph Young, who came back as an express rider from the rescuers to report on the conditions of the companies. These were the articles which I found most interesting. I am adding quotes from this material to the history I am writing.
Of course the most important contribution for what I am writing is the excerpt from the history of Isaac Wardle. The Church History site only includes those parts dealing with the handcart trek.