I just want to add some of my own experiences to this debate, with the greatest respect to the scholars above. To do this I am going to have to write about myself, so I'll make it quick and try not to be too dramatic.
The point I want to make is very simple. I do not know what a Folk Song is, but I certainly know what it isn't. There has been a lot of talk about evidence and the only evidence I have is the life I have lived. I left home at 17 and for the most part have been living on my wits ever since. At one time I was playing music in the street to survive, I have travelled round the West country, sleeping under hedges, busking to get enough for food for the next day. Yes of course I make money from Folk Songs but I don't think that demeans the art in any way. I fell in with the Gypsies decades ago learning their Art, songs and lifestyle first hand, and was taught how to earn a living with the streangth in my hands and what ever is between my ears, living half in a house and a trailer, and I married a Romany Gypsy lass.
When it comes to songs and singing the relevance of a song has to be measured against the life of the singers who hear it. If it passes that test, weather it be printed on a Broadsheet 250 years ago or composed last week, it will be sung as an expression of that experience. It doesn't mean it's somehow better or worse for that, but it does mean it may be viewed as relevant to that huge mass of musical excellence we call Folk Song. My best freinds wife (a Romany) sat me down and taught me 'The Tanyard side' face to face as she was taught by her Mother. My singing teacher and freind the late Bill House taught me how to sit, how to breath and how to project a song as he taught me 'One night as I lay on my bed' as his father taught him when Bill was six years old in 1906, the same year as his father sang it to the Hammond Brothers.
So yes-I know a folksong when I hear it, whatever it's background. It's a simple emotional recognition, that will capture your attention, make you smile in appreciation, or shake your head in sympathy. That, I believe is where in begins and ends, and it matters not how many arguments are raised for and against any academic point. Folk Song differs from other music as night does to day wrote Bert Lloyd, but when does day become or night become day? The answer is when ever you decide.
That said I still intend buying and reading the book.
kind regards
Nick