Textbooks and encyclopedias are examples of tertiary sources--sources that provide overviews of information on a topic. In the field of history, tertiary sources often provide not just an overview of the topic, but also discuss how a topic has been analyzed, interpreted, and argued about among scholars over time (historiography).
Whenever you have questions about the people, places, events, documents, and themes in the primary sources you are analyzing, subject encyclopedias should be your go-to source.
Entries in subject encyclopedias typically are not quoted from or cited as evidence in a final paper. However, the entries can help you identify scholarly secondary sources that may help you situate your primary sources in their historical context, and those sources would be cited in the final paper.
Several excellent subject encyclopedias on various aspects of medieval history are available in the print reference section on the first floor of Collins Library. A few highlights: