Oliver Wendell Holmes Library Catalog
Another type of secondary source is a scholarly journal article. These are much shorter than books, but also much more specific. You won't find a complete biography of Nelson Mandela in a scholarly journal article, but rather an analysis of a specific part of his life or work. Scholarly journal articles often have a thesis and a specific point of view, which is something to keep in mind as you read and take notes.
Here are a few databases that contain scholarly journal articles: