Reference resources help you choose, explore and develop your topic! Reference sources such as encyclopedias and dictionaries contain overviews and background information. They sometimes contain statistics, definitions and lists of resources for further study. When you have a better understanding of your topic, it will be easier to come up with the search terms you'll use to find the books and articles you need.
Wikipedia is a reference source because it is an encyclopedia. You can use it to discover ideas and links to additional resources -- but never cite it as a source in your papers! This rule exists because Wikipedia entries can be edited at any time, by anyone, and so its accuracy can never be relied upon at any given moment.
Encyclopedia of Human Nutrition by Background Information
Find background information on your topic to get a general overview. When you have a better understanding of your topic, it will be easier to generate search terms in the library catalog and databases. Wikipedia is usually an okay place to start but never cite it as a source in your papers!
Large collection of reference books providing background information on a variety of subjects. PCC offers a brief video with instructions for using this database.