Use a search engine to find original articles which may be published in freely available, peer-reviewed electronic journals on the Web. It is best to search here after any indexing databases to avoid having to sift through a lot of information. Library catalogues to find technical reports and government reports. The new information from primary sources adds to books, journal articles etc which already exist on a subject. At some point, someone may review the literature up to a certain date, summarising and evaluating the main findings.
Secondary sources cite primary sources and are usually less detailed.
Reading a recent literature review is an excellent way of getting a broad picture of a very specialised area up to a certain point in time. Scholarly books , student textbooks or journal articles which cite previous publications. Some scholarly books may also include original information. Popular literature like self-help books or articles in popular magazines such as Psychology Today or Popular Mechanics. Search your library catalogue with subject terms and the terms 'annual review ' or 'advances' to find annual review publications.