Evidence should come from a variety of sources.  Books tend to give the most in-depth background and analysis of a situation, but are often the least up-to-date sources.  Journalistic sources (e.g. newspaper articles) are often much more up-to-date, and are good sources for specific facts, but are usually thin on background and analysis, and are occasionally factually incorrect.  Journal articles are usually somewhere in the middle.  Getting information from a variety of types of sources should give you a good balance of fact and analysis, of foreground and background.

        Beware, however, that many sources have biases.  Many present arguments, and sometimes evidence, that are weighted towards one position or another.  If it seems that some of your sources are biased in this way, try to find sources with the opposite bias; this will help you to develop a balanced picture of what is going on.

        Also beware that the reliability of sources varies considerably.  Most academic sources (articles in academic journals, books from university presses) are refereed.  This means that they have been reviewed and accepted by experts in the field.  Newspaper articles have to pass through editorial and legal staffs before they are published, which cuts down on factual errors (although some newspapers have more thorough editorial processes than others).  Many websites go through no comparable process; the webmaster can put up whatever s/he wants.  You should therefore approach these sites with more skepticism than many other sources.  When in doubt about the reliability of an information source, ask your section leader, or the instructor.