Music librarianship includes people of all kinds working in institutions of many kinds--colleges, universities, conservatories, public libraries, museums, archives, symphony orchestras.
The Music Library Association offers information and support to anyone interested in the profession.
Books about music librarianship are shelved at ML 111.
Some principal resources:
Multimedia collections from the Library of Congress of digitized documents, photographs, sound recordings, motion pictures, and texts. Over 100 collections, including, for example, African-American pamphlets, Chautauqua flyers, Depression photographs, Coca-Cola advertising, and the papers of Alexander Graham Bell, to name just a few.
Parker Library on the Web is an open source digital collection of medieval manuscripts held by the Parker Library at Corpus Christi College at Cambridge University, UK. 538 manuscripts (many of them famous) are available as high-resolution full-text with extensive tagging and complete codicological descriptions and a wide variety of features which enhance study of these materials.