Use these sources to learn about the larger historical context for your topic and get an introduction to the existing scholarship.
These reference sources can be helpful for defining your topic, finding background information, and placing your topic within the broader context of scholarship on US history.
Many students start exploring topics through Wikipedia. While there's nothing wrong with that per se, it's a good idea to explore your topic through more reliable sources like reference works. They will usually offer greater depth and cite scholarly sources that you can use in your research.