The Mudcat PermaThread Basic Folk Library might be of some help. I wouldn't suggest that you rush out to get them all, but an overview of what's "respected" source material may help when you come across something and need to decide whether to add it to your resources. If nothing else, it should give you an idea of the multitude of song types that fall under the "folk" umbrella.
Like blues, folk is as much "attitude" as it is a matter of playing the "right" tunes. Some of the references cited above are "things you should know" to join into the tradition(s); but they're not necessarily good guides to what gets played frequently, at least in my area. For that, you need to observe the sessions accessible to you, and get to know the players and the "group culture" of each session.