I especially enjoy the pictures he did while he was at Fort Union at the confluence of the Missouri and Yellowstone Rivers. I've stood there and moved back and forth until the perspective in the painting matches the view. It's kinda a neat feeling to know I am standing within feet of where he stood while working.
Side note: many of the illustrations of individual items such as war clubs, clothing, decorations, knives, guns, etc are in museums. I've seen pics of the artifacts today next to reproductions of the original paintings and they are PERFECT in detail. Because he did not romanticize or exaggerate or take "poetic license", you can take his work as fact for reproducing period equipment, and know you are being accurate.