Opera is a musical form, so you may want to use the Fine Arts Library, containing MSU Libraries' art, music, and photography collections, located on the 4th floor of the West Wing of the Main Library. It has its own web page, here. There is an “Ask the Librarian” link here, too, if you want help with music resources. Here are the major encyclopedia and two periodical indexes, to help you get started.
For more music resources see the Music Best Bets page.