According to the various disciplines here are some examples of secondary sources:
Anthropology: literature reviews, journal/scholarly articles, books, and multimedia
Arts (fine): Critical interpretations of art and artists—biographies, reviews, recordings of live performances
Communications: journal /scholarly articles, online postings about the communications field
Education: journal/scholarly articles and data
Language and Literature: Literary criticism, biographies, reviews, and textbooks
Sciences: Publications about the significance of research or experiments