I spent a summer doing this in grad school for the Vegetation Type Mapper project at UC Berkeley. I'm not going to lie to you--it was a ton of work. But the results were cool. The site has all the old maps georeferenced, plus ways to download them.
Needless to say, the library was involved in the project, as was a giant scanner. We relied on ERDAS Imagine software to georeference the old maps to current USGS base maps. There was also a lot of accuracy assessment involved to make sure we minimized error in the georeferencing process. Probably one of the trickiest parts was making sure the old landmark you were using as a control point had not substantially changed in the intervening decades.
My professor and her colleagues published a paper detailing the project.