I just noticed that the most recent edition of the Traditional Ballad Index, version 2.6, was issued November 20, 2011. The What's New page will give you an idea of how the Index has evolved over the years. Here's what Bob Waltz says was added in the most recent edition:

Materials Added in this Edition

The following books were added to the Index in version 2.6:

• Horace P. Beck, The Folklore of Maine (1957)
• Charles K. Wolfe, Folk Songs of Middle Tennessee: The George Boswell Collection (1977)
• David Buchan and James Moreira, editors, The Glenbuchat Ballads (2007)
• Sigrid Rieuwerts, editor, The Ballad Repertoire of Anna Gordon, Mrs Brown of Falkland (2011)
• Gavin Greig (Kenneth S. Goldstein and Arthur Argo, editors), Folk-Song in Buchan and Folk-Song of the North-East (1963) [now fully indexed]
• E. B. Lyle, editor, Andrew Crawfurd's Collection of Ballads and Songs, Volume 1 (1975).
• Iona and Peter Opie, The Singing Game (1985)
• Harry B. Peters, Folk Songs out of Wisconsin (1977)
• James Reeves, editor, The Idiom of the People, (1965)
• Art Rosembaum, Folk Visions & Voices: Traditional Music and Song in North Georgia (1983).
• Alfred Williams, editor, Folk-Songs of the Upper Thames (1923), plus the additional material from this collection edited by Chris Wildridge.

We now have 10731 different songs, under 18211 titles. There are over 38,000 references (excluding Roud references), pointing to something like 30,000 texts, and more than 7800 recording references. Because of duplicates, we cannot give an exact broadside count -- but the number of citations exceeds 3200.

At least 2306 songs were added or had their entries updated in version 2.6.

The NOTES now include almost 1,400,000 words. There are 358 songs for which the NOTES exceed 500 words; 151 with at least 1000 words of notes; 15 with at least 5000 words of notes.

I count 215 books fully or partly indexed, with many more cited in ADDITIONAL entries. At least 827 sources are cited in the NOTES.

My goal is to possess all the books indexed in the Traditional Ballad Index. I suppose I could shoot for all the books indexed by Roud, but that's biting off a bit too much.