Oh, I almost forgot. Two more useful reference books:

"Folksingers and Folksongs in America" by Ray M. Lawless, and
"Folk music: More than a song," by Kristin Baggelaar & Donald Milton (published in England as "The Folk Music Encyclopaedia."

Both are long out of print, but scondhand copies should be relatively easy to find.

The Lawless book was first published in 1960, with an enlarged second edition in 1965. It covers most of the important singers who were active just before the Folk Boom, along with others who never became prominent. It's an excellent snapshot of the folk music scene at the time, but like the Baggelaar& Milton book, it misses some important West Coast performers.

The English title of "The Folk Music Encyclopaedia" is misleading, but the Baggelaar & Milton book does include entries about most of the important folk musicians up to the time of its publication in 1976, IF they had made records for major labels or if they had been regulars on the East Coast concert and festival circuit in the early Seventies. So, for example, there are no entries for Larry Hanks, Walt Robertson or Harry Tuft, or for many important UK singers and groups like the Ian Campbell Folk Group, Shirley Collins or Isla Cameron. Most of the entries read like they were taken from liner notes or official bios supplied by the musicians' booking agencies.