Most of the databases in the A-Z list are subscription databases. For all subscription access:
Répertoire International de Littérature Musicale abstracts of music literature is a comprehensive, ongoing guide to publications on music from all over the world, is an indispensable tool for scholars, students, librarians, performers, teachers, and music lovers. RILM includes over 620,000 records dating from 1967. Over 30,000 new records are added every year, covering all document types: articles, books, bibliographies, catalogues, dissertations, Festschriften, iconographies, critical commentaries to complete works, ethnographic recordings, conference proceedings, electronic resources, reviews, and more.
The AES E-library contains over 17,000 fully searchable PDF files documenting the progression of audio research from 1953 to the present day.