Most major composers' works have been cataloged and numbered by scholars. This information has been published in thematc catalogs (called "thematic" because they include the musical notation for the principle themes (tunes) of each work, to aid in identification).
It's from these catalogs that we get the opus numbers used to identify classical musical works. (Some composers' work numbers have become associated with the name of the scholar who cataloged his works -- for instance, "Koechel" numbers for Mozart).
Thematic catalogs also contain a lot of useful information on the primary sources (manuscripts, early editions) of a composer's works.
Cataloged by Schmieder (BWV numbers)
Call no.: MUS REF ML 134 .B1 S3 1990
Cataloged by Koechel (K. numbers, "Koechel" numbers)
Call no.: MUS REF ML 134 .M9 K55 2006
Cataloged by Hoboken (H. numbers)
Call no.: MUS REF ML 134 .H272 H6
Cataloged by Kinsky
Call no.: MUS REF ML 134 .B4 K5 1955
Cataloged by Deutsch (D. numbers, "Deutsch" numbers)
Call no.: MUS REF ML 134 .S38 D48 1978 Ser.8 v.4
For any given work in a thematic catalog, you can expect to see the following data. (Many catalogs are in German, so German terms are given here, with translations):
Title and catalog number
Komponiert (date of composition)
For vocal music:
Music incipit (the first few bars of the music); or principle theme(s)
Autograph (the composer's original manuscript)
Abschriften (others' handwritten copies)
Erstausgabe / Originalausgabe (first edition)
Ausgaben / Nachdrucke (other early editions)
Gesamtausgabe / Samtliche Werke (collected edition of the composer's complete works)
Anmerkungen (notes, remarks)
A thematic catalog may also have:
Search the Library's catalog using this subject heading:
[Composer's name] -- Thematic catalogs.
If we don't own it, you can use the same search terms to search WorldCat (a database of other libraries' catalogs, available on the Library's homepage under "Databases"), then place an interlibrary loan request.