Very glad to hear that the book's been of interest - I wrote it for people who were involved in the Folk Revival - including me - so that the way it had developed over time was more fully on record. I kept finding things that surprised me and just wanted to sort out what they meant.
I wish greater notice had been taken of Jacobs' comments. He was Australian and was regarded as rather a pushy colonial. I think he got fed up of trying to get his ideas taken seriously, so went to America where he became very influential. And though I don't agree with Maud Karpeles (or Sharp's) definitions, I think Karpeles has been greatly misrepresented and underestimated. She did some amazing work.