Secondary Sources. The most common type of source you are likely to encounter is a secondary source. A secondary source is any source about an event, period, or issue in history that was produced after that event, period or issue has passed. Aside from a textbook, the most commonly assigned secondary source is a scholarly monograph - a volume on a specific subject in the past, written by an expert. Also common are articles in scholarly journals, which are similar to monographs, but on a smaller, more focused scale.