In secondary sources, authors analyze and interpret primary source materials.
Secondary sources can be scholarly or popular. Scholarly sources (sometimes called "academic" or "peer-reviewed" sources) are written by and for experts and typically include bibliographies and citations. Popular sources are written for a general, non-expert audience and can be authored by anyone.
The History of Science, Technology and Medicine (HSTM) is the definitive international bibliographic database for the history of science, technology, and medicine and their influence on culture, from pre-history to the present. Includes records of journal articles, conference proceedings, books, dissertations, serials, maps and other materials.
JSTOR contains thousands of full-text articles from journals across the sciences, humanities, and social sciences, and in particular is known for providing online access to extensive archives of back issues of journals, and as such is particularly helpful for historical research.