I've been perusing archives of songs here in New England, esp Maine. The Maine Historical society has a number which were included in letters sent from the front. Apparently the Union government actually employed songwriters who went around the camps singing songs (mostly sentimental) and passing out the lyrics which the boys then sent home to their loved ones. If the loved ones wanted the tunes, they had to buy the sheet music!
Have also found several interesting personal songs as well..
best- Julia