Before you start looking for books and articles, take a minute to think about your search terms. Remember that most search engines aren't like Google - they won't expand on your terms or add related ones. To make sure you're being as thorough as possible, write down some synonyms for your terms, as well as related people, places, and concepts.
A mind map can be a great way to do this:
You might also want to consider keeping a research log to track your research.