When I wrote my Chickamauga article I got the idea while walking the ground, and later made sure to walk that ground again to get pictures for possible future use. I've always believed that walking the ground was vitally important to understanding what happened. A few weeks ago I received an email from a gentleman asking what sort of sources I use to plan my trips and that struck me as being an excellent blog topic, so here's some of my thoughts.
For print sources the two books I utilize the most are Alice Cromie's "A Tour Guide to the Civil War" and the Civil War Preservation Trust's guide. Cromie's guide is a bit dated and some sites don't exist anymore, plus there are new sites that are not in there. The CWPT's guide only includes the sites that paid to be part of the guide. These are all your major sites but some of the smaller (and often well done) sites are not included. Using those two will give you a good start.
A great online source is just going to the various state parks department websites and seeing what state parks there are. Most states have some sort of overview of what is at the park to see. Most sites are preserved at this local level rather than under the NPS.
Another great resource for traveling is Blue and Gray magazine. Each month they tackle one battle (or in the case of their Gettysburg issues, one aspect of the battle) and give a nice history of the battle plus an excellent tour. These tours are very detailed and often written by the park historian so they can be considered to also be fairly accurate. This magazine comes out 7 times a year and usually has one issue devoted to Gettysburg. Since its been out 20 odd years they have covered much of the war so chances are if you're looking for a particular battle they have covered it, unless it is quite small. Another caveat is that in their early days the tours were nowhere as good as they are now. The tours are quite impressive now. They also always have great maps.
Maps are another resource I try to utilize. I'll look at a period map and then compare it to a modern map to see what areas might have history that is a bit of the beaten path. Then I'll research to find out if there is a local museum or park that interprets its place in the war.
A truly great place to go is the Technical Information Center for the National Park Service (TIC-NPS). The only drawback here is that the center is located in Denver so if you're not a local your access is a bit harder. But this is a treasure trove of NPS materials, studies, etc. I used to go there all the time and browse the collection. Some of the studies the park service did are mind boggling. For instance I've twice found maps (Vicksburg and Chickamauga) that listed the size and style of every tree in the study area. Of course this didn't aid the study of the battle at all but it was fascinating that they went thru that kind of work. If there is one thing the NPS is good at it is studying something to death. It might take 20 years of study before they make a move but after 20 years they'll know more about the thing than anyone would care to know. Of course the same project done privately might have only taken a few months, but that's just how things operate at the NPS. I'm pretty sure TIC does research thru mail request, but I've never had to find that out. Incidentally if you are local and go, be prepared for tougher security than seems necessary. Maybe the NPS also has secrets on how to make atomic bombs because its the only library I've been too that requires a metal detector and an escort thru the building. Plus I was once told that before they could print anything for me they'd have to check the security clearance on the document. Somehow I don't think there are any state secrets in Shiloh maps.