The Juilliard Manuscript Collection

http://juilliardmanuscriptcollection.org/ 

Classical music fans will want to check out the Juilliard Manuscripts Collection, a spectacular collection of very rare manuscripts. These documents were donated to the school in 2006 by Bruce Kovner, a business professional and philanthropist who served as chair of the board at Juilliard. This collection includes engraved first editions of manuscripts by Johannes Sebastian Bach; an autographed letter from Ludwig van Beethoven; a copy of Beethoven's 9th Symphony - with his own annotations - that may have been used in the symphony's very first performance; and a signed holograph by Claude Debussy. Visitors to this website can search for manuscripts by composer. In addition to the luminaries mentioned above, this collection contains manuscripts by Johannes Brahms, Aaron Copland, Dmitri Shostakovich, Richard Wagner, and more. Each manuscript is accompanied with complete bibliographic information. 

via https://scout.wisc.edu/archives/r47298/the_juilliard_manuscript_collection