yeah, that's kind of the picture I get from reading country music histories that delve into old-time music's history.
The other book that I think covered this is this one:
https://www.amazon.com/Tune-Charley-Patton-Rodgers-American/dp/0807157805
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I think there's also a lot about those roots of country msuic in the Bill C Malone books, but I don't remember what was in which book- he is best known for Country Music USA but also has a book of essays called Sing Me Back Home which talks a lot about early commercialization of southern music through sheet music publishers and traveling gospel singing schools .
I don’t think that HypeMachine’s Merci Table tool was mentioned in the MeFi thread, it helps you find Bandcamp pages for artists in a Spotify playlist.
I’ve also heard of https://soundiiz.com/ but haven’t tried it (never had a Spotify account)
How about this? It's sort of black with orange and yellow and has 16 songs on it.