The Wisconsin Historical society has one of the best online digital archives available: Wisconsin in the Civil War. I am always impressed whenever I need to look something up on a Wisconsin regiment. Via their website (and a new version coming soon) anyone can look up not just demographic info, but also search through letters, diaries, newspaper clippings and memoirs. I know I have written about their excellent collections before, however, with the beta launch of their new site I wanted to mention this resource again as it is indeed one of the finest online. For example, if you’re looking for a regimental history they have them all online and organized nicely for you. See here, and here. Addtionally you can look up Wisconsin troops involvement in specific battles.