There are several sources of librettos available:
Search Tips for diction resources
In OneSearch, you can add "IPA" (International Phonetic Alphabet) to a keyword search to find transliterations of songs, arias, or complete operas.
To find books about diction, search Singing--Diction
The Diction Coach series is a collection of several aria anthologies by voice range. Each volume includes IPA transliterations of arias, word for word English translations, and CDs with diction coaches from the Metropolitan Opera and The Juilliard School reciting the text slowly, allowing the student to repeat each line.
The ABCs of Russian Diction is a DVD by Vladimir Morosan. An accompanying insert includes information on transliteration of Russian and IPS.
Phonetic Readings of Songs and Arias: Authentic Pronunciations of 413 Italian, German, and French lyrics from "The Singer's Repertoire" in International Phonetic Alphabet Transcription, by Berton Coffin.
Nico Castel's series of complete opera libretti includes the original texts, word-by-word translations, and IPA transcriptions for entire opera libretti. They are kept in the Music Collection Reference area with call numbers beginning ML 49.
Occasionally, an individual opera's libretto is published with English translation and IPA transcription, such as Smetana's Bartered Bride.
The following reference books have word-by-word translations and International Phonetic Alphabet (I.P.A.) pronunciation for important operas:
The Nico Castel Opera Libretti series (In Reference Area)
(Available on the Music Collection Reference Shelves, with call numbers beginning MT 49)