The NNU library has a good selection of books regarding composers and their lives. Look for a specific composer. Most books that fall into this category are shelved in the ML410. Books containing multiple composers can be found at ML390
Ebook Central includes 30 titles in its Opera Classics Library. Each ebook includes information about the provenance, commentary, and analysis for the opera as well as the libretto with translation. You'll find works by Puccini, Strauss, Verdi, Mozart, Wagner, and others.
You'll also find libretti in the Naxos Music Library database.
Go to RESOURCES > OPERA > LIBRETTI