I think your best bet would be to start with a couple of traditional music songbooks written for piano and vocal. That way you can get a sample of different types and topics and decide what you like, and then branch your way out from there.

I went to Mel Bay publications at:

http://www.melbay.com/scripts/bycategory.asp?category=34

Here are a few of the items at that address you might like, and many come with tapes or CD'stohear some of the selections. Of course many if not all of the melodies willalso be in the DigiTrad so you can hear and choose what you like.

Appalachian Folk Songs for Piano & Voice, by Richard L. Matteson, Jr.

Folk Songs for Schools & Camps, by Jerry Silverman

Folk Songs of the British Isles, compled by Andrew Gant

Front Porch Old-Time Songbook, by Wayne Erbsen

Music on the Mountain, by William Coulter and Barry Phillips

Old Time Gospel Songbook, by Wayne Erbsen

The World Turned Upside Down, by Barry Phillips

Doc Watson & Clarence Ashley: The Original Folkways Recordings 1960-1962