Steve,

I use Worthington Chauncey Ford. "Broadsides, Ballads &c. Printed in Massachusetts, 1639-1800," (1922), it's online (google books)

Another book is Guide to the Study of United States Imprints, Volume 1
by George Thomas Tanselle; which is partially available (preview- google books).

Both the Harvard Library and Antiquarian Society are accessible via email and telephone.

I've had trouble getting some books and MS copies from libraries here. Many of master thesis with collections of folks songs are not available unless you visit the library where it's housed. The same is true with many collections of ballads and songs in MS form.

Richie