In legal research, "secondary sources" refer to legal writing that does not emanate from the courts or the legislature, i.e. they are neither judges' decisions nor laws passed by a legislature. Secondary materials are the work of one or more authors who analyze the law from a given perspective. They include journal articles, monographs (which could be textbooks, case books, or treatises), collections of essays, government studies or reports, conference papers, or continuing legal education program materials.
Secondary sources in law depend largely on primary sources--new statutes and court decisions give rise to analysis and discussion in books and articles; likewise, legal drafters and judges may give considerable attention to what legal scholars have had to say about a particular subject. Nonetheless, legal texts and articles are "secondary" in that they are not law.