Rules writers pay attention! Mike Willegal has written an analysis of the effectiveness of black powder muskets.
Andy Watkins has this page with an amazing list of his ratings on hundreds of historical reference books. It’s worth checking before you take the plunge on a particular volume.
The Internet Modern History Sourcebook is an amazing resource for all students of history — which includes, I’d guess, most historical wargamers. I’m constantly amazed at what is available on the internet today. Materials like this would take up many, many square feet of shelfspace in my little study.
Historical miniature gamers can never have enough research material. Here is a huge collection of primary source material on the United States from the Antebellum through Reconstruction. This University of Michigan digital library has 8,500 books and 50,000 journal articles with 19th century imprints. The collection should be of particular interest to Civil War and Mexican American war gamers.
Mikko Kurki-Suonio at Small Cuts has an article about the true (economic) costs of miniature wargaming.