There are many libretti of operas and musicals in the Music Library. To browse these, try the Subject Headings:
Musicals - Librettos
Operas - Librettos
Nico Castel has published an important series of opera libretti with IPA transcriptions and translations. To view this series, click here.
Browse under the Subject Heading "Singing - Diction" to find books on diction for singers, the International Phonetic Alphabet, and related topics.