A primary source is the work itself being discussed, and also, in research, materials written or created about it during the time period in which it was created. Primary works can include music scores, manuscripts, concert reviews, diaries and letters of the composer and his/her collaborators, interviews, and speeches.
You can find books and articles and limit their publication date to your composer's/work's time period using SuperSearch. JSTOR is especially helpful because it offers historic coverage going back to first issues of journals. So you can find 1845 editions of The Musical Times and Singing Class Circular and reviews of Handel's Messiah played for "the aged and distressed housekeepers."
These include critiques and analysis not from the lifespan or shortly thereafter of a composer, or more broadly, the time period. Journal articles and books that provide overviews fall into this category. Analyses of a particular work over time- treatments detailing a cultural or political aspect, offering the author's interpretation are secondary sources.
A History of Western Music by You can search any music time period across the library's collections.
