Please, please, please learn to question such statements as:

"In the old days itinerate harpers would pass on tunes they learned while learning new songs when they went from town to town".

They (the itinerant harpists) may have done, at certain times and in certain places, but they may not have had an awful lot to do with folk music! One of the reasons why the terms 'folk music' and 'folk song' are so hard to define is that people will insist on making things like this up.

Still, it's probably better than the marketing man's definition which is: "anything acoustic but excluding 'country & western' and 'blues' but including 'folk rock'". In the UK the marketeers also include under 'folk' things like military bands - which don't fit in any other of their cretinous categories.

As you may be, possibly(?), primarily interested in American songs you may want to look for a book called: 'Dear Companion: Appalachian Traditional Songs and Singers from the Cecil Sharp Collection' compiled and edited by Mike Yates, Elaine Bradtke and Malcolm Taylor, published by English Folk Dance & Song Society, 2004 (ISBN: 0 85418 190 3). And you may also might like to look out for recordings of traditional singers and musicians made by your great countryman, Alan Lomax.