The Library of Congress classification system organizes books by topic, which makes it easier to browse for things that interest you. The ML call numbers are for books about music and music history, and the MT numbers are for instruction and study of music. Try these areas for books of interest to composers:
ML385-429 Biography
ML430-458 Composition and performance
MT40-67 Composition
MT68 Improvisation. Accompaniment. Transposition
MT70-74 Instrumentation and orchestration