I Hear America Singing. I cannot emphasize how much we love this book and CD. The CD sat in our car’s player for about 4 years without being rotated! Classic American folk songs appropriate for kids, but performed at a high level and enjoyable for adults.
If you really want to dig into American folk, the best resource is Smithsonian Folkways.
By Period, on Naxos
Free online music grouped by composer: MusOpen
Hildegard von Bingen (art: Mandalas)