There are a lot of music reference books out there from research guides to literature/repertoire guides, basic history to theoretical treatises. Print is stll alive!
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The Library of Congress call number system is an "address" for each item. It is used by most academic libraries and applies across formats. Call numbers look like this: M1001 .B46 op. 3 no. 2 or ML410 .C545 H6 or MT265 .P42
Here's what that means...
The first letter designates that the item is a score, music literature (biography, history, criticism, philosophy) or music theory/teaching (how-to's, theoretical descriptions, treatises).
"M" = score, "ML" = literature, "MT" = theory/teaching
The second grouping is a "Cutter" number...a series of letters+numbers in a type of cipher.
For example: Chopin's Cutter designation is C54 or C545. When you see this combination (as above), the item is by/about Chopin.
Other helpful tips:
M3 is the collected works section. These are curated complete works by one composer. Solid scholarship.
Score designations: M2## are duets or accompanied solo works, M3## are trios, M4## are quartets, etc.
Print music resources that YOU should know as a music scholar. Familiarize yourself with these resources and you will have an easier time of researching!