Mr. Gatling's Terrible Marvel by Julia Keller
I bought it for $10.49 @ Costco today. It confirms Ben "Beast" Butler used the guns on some Rebs who were peacefully bartering with the Yanks. It's a small paperback that has a lot of pictures, endnotes, bibliography and an index. It mentions the post-Civil War career of the gun along with Custer, Buffalo Bill, Teddy Roosevelt, it's use in Africa by the British Army. I didn't see (or rather find) any mention of Gatling adapting his gun to be steam powered and electric power. The book is stronger on the story of Gatling than it is on the gun. Still, at a glance it appears very promising.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!