If you're looking for more in-depth/book-length studies on music in general or of a particular kind of music, try searching the Five College Libraries Catalog or WorldCat. Try a combination of keywords like
You can narrow further to a particular time period by adding keywords like "20th century" or to a specific genre by adding keywords like "traditional" or "ballads."
If you have a particular literary figure in mind and you'd like to find out if anyone has written about that author and how his/her work relates to music, you might also try searching on that author's name and music or a particular kind of music. For example,
kate o' brien opera
or, more broadly,
ireland literature music
Keep in mind that library catalog records are only a brief synopsis of the book they represent. Sometimes you need to get your hands on the book and flip through the table of contents and index to find out if it covers the particular author or literary work in which you are interested.
Click on in the catalog to have an item sent from another Five College library to MHC. You will get an email when it arrives. Pick it up at the MHC circulation desk and return it there when you are done.