Can I also recommend John Lomax's 'Adventure of a Ballad Hunter'
Dated in places, but extremely enjoyable and evocative of the time and the period.
I think it was there I read of Leadbelly, who was acting as driver for their collecting trips, curing a raging toothache with a pair of pliers waiting outside a Texas prison (the authorities had refused hi permission to enter the prison to assist).
Somewhat later, Alan Lomax's 'The Land Where the Blues Began' needs no mention, surely?
Some of the earliest collectors and anthologiists, Scott, Motherwell, Maidment, Hogg, Ritson..... are dealt with superbly in Sigurd B Hustvedt's 'Ballad Books and Ballad Men' (Massachusetts, 1930) - indispensable for anybody interested in ballad and song backgrounds   
We got ours after a bit of a search - don't know the situation nowadays
Jim Carroll