The liner notes to the wonderful CDs first issued by Folkways as 'Old Time Music With Clarence Ashley' and then reissued as 'The Original Folkways Recordings 1960-1962; (mentioned above in guest Hoot's post) should be downloadable for free from Folkways.
(Though I don't understand why anyone who likes American old Time Music would not want the CDs as well!)

   In the notes Ralph Rinzler writes about meeting Clarence Ashley, discovering Doc, picking banjos in the back of a pickup truck with him and later driving across America chatting and trading tunes with him. And what it meant to Doc and Tom Ashley to have their home-music appreciated when they thought it no longer had an audience outside their own homes in their home states. This is Ralph's account, his words, not Doc's, but it to me it seems full of honesty, respect, integrity, friendship and self awareness. The CDs are fantastic of course, not just for the music, but also the pictures and these accounts of the recordings and Rinzler's experiences making them and spending time with the musicians. I think this is probably a better starting point that the abovementioned book.