There's an extensive essay by Barry O'Connell which covers, amongst other things, Dock Boggs' banjo playing style. It's available for download from the Smithsonian Folkways website. It appears in an abridged form on the CD reissue of Boggs' Folkways albums from the 60s, but you can get the full text here: "Down a Lonesome Road': Dock Boggs' Life in Music"

It's a pdf file.

Cheers

Nigel