ML books contain literature about music. This includes reference books, like dictionaries, encyclopedias, and bibliographies, as well as history and criticism.
ML410 is the single call number that takes up the most shelf space. These are biographies of composers, including analyses of composers' works.
MT books include music study and instruction, including music education, music theory, and methods for teaching. Notice that bound periodicals (older issues of journals and magazines) are found past the MT books.