These four terms are interrelated, and there are sources that are distinguished for what kind of text and what kinds of information that you want to know. Individual publications may combine any of these aspects. The influential Castel librettos (see below), combine librettos to entire operas, along with IPA transcriptions, word-for-word translations, and poetic translations when the meaning of the word-for-word translation is difficult to understand. Some librettos and song text collections will not have any translations from the original language.
Song Texts vs. Librettos: Collections of song texts refer to the texts for individual songs, mostly independent art songs, but may come from an opera as well. Librettos refer to the words of a dramatic, sung work, such as an opera, a musical, or an oratorio. In musical theater, these are often called "books." (Libretto literally means "little book" in Italian). There are publications of song text collections and librettos as books, separate from the score.
IPA (International Phonetic Alphabet): The IPA is a way of describing the sounds of an individual word or syllable. These can be useful for languages that you are unfamiliar with. Some scores come with IPA transcriptions under the text or at the front of the score. There are also resources that are separate from the score with IPA transcriptions, such as IPA Source and some libretti.
Singing Translations vs. Poetic Translations vs. Word-for-Word Translations: Not all translations are equal. Singing translations allow you to sing a song or aria in another language, making sure there are correct number of syllables (more or less), that certain words are emphasized to go along with the musical setting, that it sounds natural in the new language, and may keep the original rhyme scheme. Poetic translations try to convey the meaning of the text as close as possible, while still sounding natural, or at least poetic, in the new language, though they may not try to keep the same number of syllables, match the meter, or rhyme. These translations are primarily for audiences to follow along. Word-for-word translations provide the translation of each word, so that the singer can understand the exact word that they are singing, not just the sentiment of the entire phrase.
The series of opera translations and transcriptions by Nico Castel are the gold-standard in music scholarship. They provide background information on the works and word-by-word and poetic translations in addition to IPA transcriptions of authoritative editions of the texts of operas and art songs. You can find them in the Performing Arts Collection reference section between ML 48 and ML 50.
Other resources are available for art songs, often organized by language or individual composers. Here are some other prominent examples: