Here's a brief guide to some of the important Library of Congress call number areas in Music Literature (ML Class):
ML50: Opera Librretti
ML100: Music Dictionaries and Encyclopedias (general)
ML102: Music Dictionaries and Encyclopedias (arranged by topic)
ML110-111.5: Music Librarianship
ML128: Music Bibliography (arranged by topic)
ML134: Composer Bibliographies
ML159-161: Music History (general)
ML410: Composer Biography
ML416-ML429: Performer/musician Biography
ML3506: History of Jazz
ML3795: Music as a Profession/Vocational Guidance
ML3797-3799.5: Music Research
MT: Music Instruction and Study