I visited with a PhD military historian about 35 years ago. Nothing like being with somebody who knows all about everything. Even the terrain. Stand somewhere and he'd say, "General XXXX" was here with his staff" or "Here's where XXX soldiers met at bayonet point." If I were wealthy and wanted to learn some stuff deeper than a brochure, I'd go to a good university and hire a PhD grad student or post-doc interested in the history of that particular battle or campaign. Hire her/him for a couple of days at a good rate and pay cash. Pay for a day or two of research so the historian would have time to organize and provide you with copies of this and that. I'll bet for $2K you would get a hell of an education.