Secondary sources are publications that describe, comment on, or analyze the law. Dictionaries, encyclopedias, law reviews, and books are all secondary sources that can be helpful in the early stages of research, especially when you know very little about your topic, by providing overviews of unfamiliar subjects, introducing key terms, and including citations to important primary sources.
Judgement in the Underworld, from the Book of the Dead, ca. 350 B.C.