Here's a brief guide to some of the important Library of Congress call number areas in Music.
And here's a link to the Library of Congress M Class guide
M: Music scores and music recordings are generally classed as M.
M2: Authoritative scores/editions
M3: Composers' complete editions
M20-M39: Piano music
M40-M59: Stringed Instruments
M60-M111: Wind/Brass instruments
M177-M990: Music for 2 or more solo instruments
M1000-1160: Orchestra/Chamber Orch./String Orch. (including concerti)
M1500-1527: Secular vocal music--Dramatic music (Operas, musicals)
M1528-M1998: Secular vocal music (other-Songs, etc.)
M1999-M2199: Sacred vocal music
ML50: Opera Libretti
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
ML156: Dicsographies
ML159-161: Music History (general)
ML410: Composer Biography
ML416-ML429: Performer/musician Biography
ML800-ML927: Stringed Instruments-History
ML929-ML990: Wind Instuments-History
ML1700: Opera, Musical Theater-History and Criticism
ML1711: Opera, Musical Theater-United States-History and Criticism
ML2075: Film Music-History and Criticism
ML3506: History of Jazz
ML3795: Music as a Profession/Vocational Guidance
ML3797 : Musicology (general works)
ML3798: Ethnomusicology
MT: Music Instruction and Study AND Music Theory