My fine workplace, the Library of Congress, has an American Memory collection of his very folksy correspondence from 1940-1950 with staff from the Archive of American Folk Song. You can read an essay about his life and times, or you can browse the letters, which are great fun. The handwritten ones are difficult to read -- click on the "Transcription" link at the top of the page for a text version.

From a 1942 letter to the Library:

I just got the copy of my song book that you printed up . . . it is about the neatest thing that ever had my name on it . . . Is it handy there for congressmen and senators to come in and sing? I hope they bring their fiddles and guitars around and hit off a few of the most radical tunes. They are awful easy to sing, and you can sing them drunk or sober, it dont matter, just a matter of personal choice. I tried them both ways. The senators, too. You can elect just about as good a one one way or the other.