For an overview of a broad topic, try books from the library's catalog. You can search here:
If you have a more specific topic and you're looking for articles, try a search from the library's article search tab above. This will return scholarly, popular, and trade articles. Google Scholar is also a good source for scholarly articles.
If you're exploring policy or are interested in research from think tanks, try the Harvard Think Tank search.
Google is a good bet for .gov, .mil, and .edu results. you can also specify that Google search only these domains in a Google Advanced Search.