There's a lot of stuff on both Google Books and www.archive.org (I think I've got over 3Gb of downloaded books from these - over 200 titles).

Not unexpectedly (because of copyright), there's a lot of 19th century song and ballad related material, also some earlier collections too. There are a few 20th C (early mostly, but a few up to the 50s seem to have slipped through). (I've also downloaded some jazz-related material from these too).

There have been many references posted in various Mudcat threads to particular titles on both Google Books and Archive (select Text source for books on Archive); I'm surprised you haven't found any references to it. Google is (for me at least) usually faster for downloading. Archive varies a lot; if I download from there in the morning (UK) it's usually not bad. Later in the day it may become too slow to be worthwhile waiting (after the US wakes up I imagine!).

Mick