Reference sources help establish basic information, such as the definition of an unfamiliar music term or the dates of a composer's life. These are not generally considered scholarly sources (with the exception of the Grove Dictionary of Music and Musicians and the Garland Encyclopedia of World Music) and do not necessarily need to be cited or quoted from in research papers or projects.
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 Library reference section between ML 48 and ML 50.
Routledge has published a series of Research and Information Guides for Music, mostly around individual Genres, Instruments, and Composers. These guides provide excellent background information and describes important scholarship and perspectives for new research. Here are some examples:
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