Use the index (or search) these books to identify entries related to your topic or areas of interest. You might not find entries that narrowly focus on your topic; if so, try to find entries that relate broadly.
Use reference books to:
Carefully reading book reviews can give you a sense for what scholars have written about a particular topic, event, or issue. Rather than just finding individual articles or books, you can use book reviews and review articles to get an overview of the conversation among scholars on the topic that interests you.
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.