Add to the books list "Die historischen Volkslieder der Deutschen", by Rochus von Liliencron. 5 vols, mid-19th c. and thus clearly Nazifrei.

Also there are probably copies still available of the Volksliederbuch für Männerchor, 2 vols, publ. C.F.Peters Leipzig, 1906, and the corresponding ...für Gemischten Chor, 2 vols, same publ., 1915. They were re-printed by the Nazis, but as far as I can tell (mine are the reprints) the reprints are facsimiles that weren't interfered with.

Did someone already post this site and I missed it? If not: http://www.liederlexikon.de/lieder/index_html/

I lived and worked in Germany thru most of the '60s, and our local pub was taken over once a month by the local Gesangverein, to the great enjoyment of everyone who managed to squeeze in to listen. Their repertoire was all the old songs, including the most beloved chestnuts ("Anke van Tharaw", "Ach, wie ists möglich denn", "Freut Euch des Lebens", etc.) The first non-oompah songs I ever heard, I heard there.

Thirty years ago, working in Texas, I accidentally discovered that the Texas State Fair has a Deutsches Tag (many German ex-pats in Texas), so naturally that's when I went. Every year, they'd a group of Bundeswehr soldiers who'd come over from some training base in west-Texas. And they'd sing the old songs all day in chorus! It was *very* nice.

(I also had the amusing experience, one year, of realising that the two "good ol' boy" farmers sitting nearby who spoke eye-wateringly awful German were in fact native speakers who only needed a dozen bottles of beer each to regain their native fluency)