Keywords:
Subjects:
Use composers as Subjects:
Composer catalog numbers:
Mozart: Kochel = K
Bach: BWV or S (for Schmieder, who cataloged it)
Vivaldi: RV
And, of course, the opus (op.) for many other composers.
Subject Searching: (you don't need to add the dashes)
These are Library of Congress Subject Headings (LCSH) which are relatively specific and you need to learn the basic anatomy. They generally go from general to specific in a text string. Here are some examples.
Use composers as Subjects and look in results for things like "Criticism and interpretation" OR "Correspondence"
And don't forget you can use composers as "Authors" to find their pieces but also their writings, like letters, autobiographies, writing on musical style, etc
Here are lists of Madrigal scores and recordings. Note: some are online resources and some are print and located in Trible Library
Monteverdi madrigal translations. Limited number of works here.