Most of UNM's music scores and books about music are found in the Fine Arts and Design Library. When you locate a book on the shelves, look around that book for other books about the same subject. Music scores all start with the call number M, books about music history start with the call number ML, and books about music theory start with MT.
To find books, use the search box on the library front page. This searches Worldcat, the library database of books and articles. Once you find a source or two that look really good, note the SUBJECT links in the detailed record. Follow these links to find other books with the same links. If you know exactly what you're looking for, try searching for the title using quotes. For example, "Text and Act: Essays on Music and Performance".