RISM: International Inventory of Musical Sources after 1600
From the website: The Répertoire International des Sources Musicales (RISM) is an international, non-profit organization with the aim of comprehensively documenting extant musical sources anywhere in the world.
Cataloging musical sources is financed and carried out by various national and international institutions.
Independent national working groups at libraries and archives in many countries worldwide catalog historical musical sources: music prints, music manuscripts, libretti, and theoretical writings about music. The results are edited and published by RISM.