Burke, your first book is a real treasure, not least for Gertrude Degenhardt's drawings. (She has also done several album sleeves.) Unfortunately, the book has not been listed in the publishers' catalogue for a year or two, so I presume it's out of print. For the last book, you could try the spelling Zupfgeigenhansl, as that was the way the original songbook was spelled, I believe.
Others, like 'The Small Fat Songbook' by Heide Buhmann and Hanspeter Haeseler (mentioned by Wolfgang above), should be difficult to find on the Net. This book was a labour of love and, as far as I know, is sold mostly by the authors, not by a commercial distributor.
There is also a 'Folk-Lexikon' by Kaarel Siniveer which lists a lot of people - not exclusively German - omits even more and ends in 1980; 'Deutsch-Folk - Searching for the lost tradition', a slim volume by Florian Steinbiss which tries to explain the roots of the German folk revival in the British and American revivals before it and touches on topics mentioned here (interesting, but by no means exhaustive); 'A History of Folk Music' by Juergen Frey and Kaarel Siniveer; and collections like 'Raeuber- und Landsknechtslieder' (Songs of robbers and mercenaries) by Wilhelm Scherf, not always reliable as to facts; collections of women's songs, ecological songs or protest songs from the German anti-nuclear movement.
You might have a look at the DVA's website - the German Folksong Archive, housed in a charming 19th century villa at Freiburg in the Black Forest. Some day I'll have to move there! As you can see frióm their website they don't confine themselves to German material - they've just purchased Dave Harker's library!