Hi, Mysha - if you put index in the Filter box on the main Mudcat page and set the date back to "all," you'll see all our Index threads. This thread (click) should keep you busy for a long, long time.
Most of the books listed in that thread are out of print - but many are available for download at archive.org or books.google.com
For the most part, we intentionally did not index the books already covered by the Traditional Ballad Index.
Hope that helps.