I don't know the groups you are referring to but I get the idea that we are using the word "folk" here to describe a sixties kind of singer/songwriter/folk/revival sort of music. There is some good music there to be sure but my inclination would be to go beyond that to earlier folk forms and learn closer to the source. Folk music is such a broad term. Like a person who is first getting into jazz it can be daunting. My advice would be to possibly start with some folk samplers like the Putamayo label etc and sample folk music from around the world. For more american stuff browse the Alan Lomax recordings, oldtime-fiddle/stringbands, early blues, shapenote singing, early gospel, western swing and even the earliest jazz recordings such as Louis Armstrongs Hot Fives and Sevens, The Jean Goldkette Band and Jelly Roll Morton. The idea is to build a musical foundation which will inform your folk playing and inturn, all of your playing. For more modern listening try Charlie Hadens "Steal Away" album. There along with Hank Jones you will find some stripped down folk songs played by two of the best, most restrained, tasteful players in the business.
