I came across this not too long ago. It's only 44-pages but provided some perspective to what was happening around Lincoln and his security. It is written by members of the 7th Ohio Independent Cavalry.
From NARA: https://ia600900.us.archive.org/15/items/lincolnsbodyguar00mcbr/lincolnsbodyguar00mcbr.pdf
A section I enjoyed.... The next morning I witnessed an interesting scene. Mr. Lincoln came out and started toward the department, apparently absorbed in thought. The infantry sentinel presented arms as he approached, but Mr. Lincoln walked by without returning the salute. The soldier re-mained standing at a present arms. When Mr. Lincoln had passed him nearly or quite two rods, he suddenly stopped, turned clear around, lifted his hat and bowed. His manner was significant of his kindly nature. It was that of one gentleman apologizing to another for an unintentional slight. Mr. Lincoln was not a military man, yet his position made him the commander-in-chief of the army and entitled him to military honors. He understood that the duty of an officer to return a salute was as imperative as the duty of the soldier to give it. The humblest private in the ranks is entitled to have his sa- lute returned, and a failure to return it is an afifront and a breach of military courtesy. When Mr. Lincoln real- ized that he had failed to recognize the salute at the proper time he was not content merely to return it, but in his manner of returning it tendered an ample apology. (page 27)