I'm in Missouri right now working on a book on Guerrilla and Partisan Ranger Tactics in the American Civil War for Osprey. Most of the time I'm buried in books, original documents, and photographs in the State Historical Society of Missouri, which has the best collection I've ever seen in a state historical society. Their Civil War section is huge, of course, Missouri being in many ways the flash point for setting off the war.
One thing I'm struck with time and again when I'm working there is the sheer volume and value of titles coming out from the small press. The Civil War west of the Mississippi (the Trans-Mississippi Theater to you military buffs) doesn't get much play outside of the academic press, and so if it weren't for publishers like Two Trails and Camp Pope, a lot of valuable studies would go unpublished.
Midlist Writer is dedicated to the trials and tribulations of the unknown professional, but dedicated amateurs write some fine history books too. Nobody is going to pay an advance for a book on the Civil War diary of Sergeant James P. Mallery or the story of Absalom Grimes, a Confederate Mail Runner, but these stories are worth telling, and they really help out people writing books on the bigger picture!
So if you're an enthusiast on some narrow topic, don't forget to support the small press, because they might be the only ones researching and publishing about the topics you love. You can find some great novels from the small press too, so the next time you're looking for a good read, think small!