Books and music published before 1923 (I believe) belong to public domain, as well as a host of other authors whose copyrights were not renewed. As near as I know, all Civil War photographs are in public domain, unless they were retrieved from a book, database, etc, which has been copyrighted since that time.
Pine River Boys, Co I, 7th Wisconsin
"We're... Christians, what read the Bible and foller what it says about lovin' your enemies and carin' for them what despitefully use you -- that is, after you've downed 'em good and hard."
-Si Klegg and His Pard Shorty