ML54.6 - history, interpretation, translations.   For example:
     Draayer, A Singer's Guide to the Songs of Joaquin Rodrigo  (REF ML54.6.R632 D73x 1999)
     Johnson and Stokes, A French Song Companion  (REF ML54.6.J76 F74 2000)
     Phillips, Lieder Line By Line  (REF ML54.6.P55 L5 1996)
     Rohinsky, The Singer's Debussy  (REF ML54.6.D42 R62 1987)
  Schoep and Harris, Word-by-Word Translations of Songs and Arias (REF ML54.6.W65)

ML128.S - bibliographies of vocal repertoire.  For example:
     Abromeit, An Index to African-American Spirituals for the Solo Voice  (REF ML128.S4 A27 1999)
     Maroney, Music for Voice and Classical Guitar  (REF ML128.S3 M37 1997)

The New Grove Dictionary of Opera  (shelved on Ready Reference)

The New Kobbé's Opera Book  (shelved on Ready Reference)
 
 

M1497 - M1998 -- Secular vocal music
      M1500 - Operas, full scores
      M1503 - Operas, piano-vocal scores
      M1528 - M1529 -- Duets, trios, etc.
      M1530 - M1537 -- Choruses, mixed
      M1538 - M1542 -- Choruses, men's voices
      M1543 - M1546 -- Choruses, treble voices
      M1619 -- Songs, collections of 2 or more composers
      M1620 -- Songs, collections of 1 composer
      M1627 - M1853 -- National music, classified by country
      M1990 - M1998 -- Children's music, including pedagogical series
M1999 - M2199 -- Sacred vocal music
      M2000 - Oratorios, full score
      M2002 - M2003 -- Oratorios, piano accompaniment
      M2010 - M2017 -- Service music, classified by religion, denomination, etc.
      M2020 - M2036 -- Choruses, cantatas, etc.
      M2101 - M2114 -- Songs for one voice, including arias, solo cantatas, etc.
      M2115 - M2146 -- Hymnals
      M2147 - M2188 -- Liturgy and ritual
      M2190 - M2196 -- Sacred vocal music for children
 
 

ML=HISTORY & CRITICISM
ML48 - ML54 -- Librettos for operas, cantatas, etc.
ML421 - ML422 -- Biographies of singers and vocal performing groups
ML1400 - ML3275 -- Vocal music history
      ML1460 -- Vocal technique
      ML1500 - ML1554 -- Choral music
      ML1600 - ML2881 -- Secular vocal music
      ML2900 - ML3275 -- Sacred vocal music

MT=INSTRUCTION AND STUDY
MT95 - MT115 -- Analyses of operas, oratorios, etc.
MT821 -- Physiology and hygiene of voice
MT825 - MT853 -- Systems and methods of singing
MT855 - MT883 -- Special techniques
      MT883 -- Diction
MT898 - MT949 -- Techniques for children
MT955 - Opera production