Call numbers for major categories:
Piano
M 21 Collections: more than one composer
M 22 Collections: one composer
M 23 Sonatas
M 24 Suites
M 25-31 Other original pieces
M 32-39 Arrangements, transcriptions
M 200-216 Four hands; Two pianos, etc.
M 1006-1011 Keyboard & orchestra (concertos, etc.)
Organ
M 7 Collections
M 8.5 Symphonies
M 10 Fugues (w/ & w/o preludes)
M 14- Music for religious services and occasions
For more information, and sources for free online scores, see the Scores tab above.
Books about piano, harpsichord, etc. are at these call numbers:
Selected library resources and websites:
The Piano by 
The Harpsichord and Clavichord by 
Guide to the Pianist's Repertoire, Third Edition by Books about the organ are at these call numbers:
Selected library resources and websites:
The Organ by 
Dictionary of Pipe Organ Stops by Organ Literature by