Rhiannon -- my primary teacher -- has recently published a book of memories, stories and exercises, devoted to this practice. It is called Vocal River, and can be purchased at Amazon or through http://www.rhiannonmu.... That site contains samples of her music and performance video links as well.
Rhiannon also has several CDs that are primarily improvisational and wonderful to listen to – they give a sense of the richness of this art in performance -- as well as several where she sings her own through-composed songs and those of other people. She also has a CD called Flight that contains exercises you can sing to and practice with! These can be purchased through her website, or any standard music site.
My good friend and collaborator Debby Boland Watt has her own musical website, and it includes a calendar of song circle opportunities which she sponsors – see http://debbybolandwat.... This site also contains Debby's music.
Debby Watt also has a Facebook site called Sing Your Soul Alive where she posts her song circles as “events.” https://www.facebook..... One circle is a class through the Experimental College on Tuesday evenings, which Gayle and I both usually attend, and there is room in my car for someone else if you want to join us. This class is a wonderful opportunity to delve more deeply into the skills of improvisation with the same people.
There is a worldwide Facebook group called Vocal Improvisation devoted to this practice! On March 20th, there were groups singing in Germany, New York, Olympia and British Columbia, as well as other places. Sometimes people post videos of improvisations they do on this site. https://www.facebook..... Ad there are discussions that are fun!
Two wonderful and very readable books which give a strong sense for the long history of music, singing and dance – and, not incidentally, improvisation -- are:
The Singing Neanderthals
Music, Society and Education tp://www.amazon.com/Music-Society-Education-Culture/dp/0819563072/ref=sr_1_1?s=books&ie=UTF8&qid=1395429032&sr=1-