Andy Watkins has this page with an amazing list of his ratings on hundreds of historical reference books.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 British Museum has a very nice presentation on the art, culture and …. warfare! … of ancient Mesopotamia.
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.
The British Battles website has accounts, maps and orders of battle for dozens of famous battles involving the British army. Included are Battles from King George’s War, the Jacobite Rebellion, The French and Indian War, the American Revolution, the Peninuslar War, and other Napoleonic Era battles. It’s an incredible resource for the miniature wargamer.
Fields of Fire is an online community for Vietnam wargamers. The site has a lively discussion forum, and focuses both on miniature wargames and on role playing games. Among the games supported are the classics Recon, from Palladium and RAFM’s Charlie Company, both of which I own and have enjoyed.