Old newspapers of the period are a useful source too, although not as easily accessible as the Internet and subject, of course, to the bias of the particular newspaper.
You mentioned Fort Pillow in your next post. One of my local libraries has microfilns of old newspapers. I've found what two Memphis papers said about Fort Pillow right after the battle. One quoted a Union officer saying that General Forrest shot one of his own men for refusing to give quarter to Federal troops. That sort of thing often gets left out of history summaries.
The other Memphis paper, one with a decidedly pro-Union slant, mentioned that many Federal soldiers were killed inside the fort after surrender.
Neither paper mentioned that black troops kept firing on Confederate troops during the surrender negotiations under a white flag before the final storming of the fort. During those negotiations, Forrest pointed out the superior number of Confederate troops and ammunition and twice offered the Federals the opportunity to surrender. The Feds were badly outnumbered and should have surrendered.
Neither paper said anything about Federal prisoners surrendering during the battle, then later picking up their arms and fighting again, though that apparently happened. Of course, if my fellow prisoners were being killed after surrender, I'd probably pick up my rifle again too.
I read somewhere (book? web?) that some the Federal troops involved at Fort Pillow had been harassing the families of the Tennessee Confederate troops. If that is correct, then there may well have been some personal payback involved in the actions of some of the Confederate troops.
Hard to know what really happened. History is complex, and it helps to read both sides.