Check out especially The New York Public Library for the Performing Arts, located at Lincoln Center.
Apply for a library card on their website or using their SimplyE ebook app. NYPL's collections and digital resources are free to those who live, work, or go to school in New York City.
The have a huge assortment of digital resources, many of which you can access from home.
NYPL's online catalog can be confusing. Moreover, most of the holdings from before about 1978 are not in the online catalog. If you can't find what you need, talk to a librarian (Music Division: 212-870-1650).
The catalog includes holdings of the NYPL Research Libraries (i.e., the non-circulating collections) added since 1972 and incomplete listings for earlier materials. For holdings not in the online catalog, you must consult the print catalogs available in all of the Research Libraries. In this, as in any large research library, you must also speak to a librarian to be sure you've exhausted all possibilities.
The catalog also shows the holdings of the Branch Libraries (i.e., the circulating collections). You can request that any item be sent to your local branch. The Morningside Heights branch is the one most convenient to the Manhattan School of Music. Anybody who lives, works, or goes to school in New York City may get an NYPL library card.
An MSM librarian can obtain a pass for you to gain entry into the following local libraries, and, indeed, to almost any other located in the Metropolitan New York area. You can consult materials on the premises, but will not be able to check items out.
Library Finder
USA.gov: Find Libraries and Archives
Internet Archive
Digital Public Library of America
Explore more than one billion holdings in libraries around the world by searching OCLC. It comes in two versions
OCLC WorldCat
WorldCat is free and available to you anywhere in the world. But there is a caveat: It's information is not comprehensive. In particular, many smaller libraries are not represented. MSM is, however, and by following this link (notice "msm" in the URL) you will see our holdings first.
OCLC FirstSearch
FirstSearch lets you search the complete OCLC database (unlike WorldCat, above). It is, however, only available on the MSM campus.
ArchiveGrid
"ArchiveGrid includes over four million records describing archival materials, bringing together information about historical documents, personal papers, family histories, and more. With over 1,000 different archival institutions represented, ArchiveGrid helps researchers looking for primary source materials held in archives, libraries, museums and historical societies."
KVK: Karlsruher Virtueller Katalog
Search the catalogs of many of the world's major libraries all at once or individually.
LibDex: The Library Index
A portal to library catalogs around the world.
LibWeb: Library Servers Via WWW
8000 pages from libraries in 146 countries.
The WWW Library Directory
"Currently indexing over 8800 libraries and library-related Web sites in 130 countries."