The following reference books, all published by Garland Publishing, are probably not for general use but useful and informative, so they are on the list of my basic folk library. The first 4 are bibliographies with short annotations.
Terry E. Miller, Folk Music in America: A Reference Guide (1986)
James Porter, The Traditional Music of Bitain and Ireland: A Research and Information Guide (1989)
Mary L. Hart, Brenda M. Eagles, and Lisa N. Howorth, The Blues: A Bibliographic Guide (1989)
W. Edson Richmond, Ballad Scholarship: An Annotated Bibliography (1989)
Florence E. Brunnings, Folk Song Index: A Comprehensive Guide to the Florence E. Brunnings Collection (1981)
Norm Cohen, Traditional Anglo-American Folk Music: An Annotated Discography of Published Recordings (1994)