His three big novels so far (Cain at Gettysburg, Hell or Richmond, and Valley of the Shadow) feature historical characters, both great and small, on both sides of the conflict. He gets into their heads, gives them flesh and bone and heart, makes them come alive. Each book is about one particular battle or campaign, through the eyes of participants on both sides.
The books he wrote under the name Owen Parry are a little different, since they all have one narrator, a fictional Welsh immigrant who has been thrust by events into a sort of detective role for the Union army. Indeed, this narrator and his unique voice is the main reason to read the books. All his little Welsh turns of phrase are enchanting, and he has a viewpoint shaped by his own troubled past (as a British soldier in India during the Sepoy Rebellion) and his devout Methodist beliefs. Though this character, Abel Jones, now a U.S. soldier, is obviously on one side of the conflict, you will see, through his eyes, plenty of evil on the Union side. Indeed, this is what I like most about Peters, other than his compelling prose: He is very honest about human nature. There is good and evil on both sides, and in each individual.
The books by Ralph Peters writing as "Owen Parry" (the Abel Jones series) really should be read in order, which is as follows:
1. Faded Coat of Blue
2. Shadows of Glory
3. Call Each River Jordan
4. Honor's Kingdom
5. Bold Sons of Erin
6. Rebels of Babylon
(The last in the series is the only one I haven't read yet, because I dread finishing this series, which I wish would just go on and on. I keep putting off that last book, to be relished at some point as a special treat for myself!)