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). Most entries also provide short bibliographies of sources for further reading.
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.
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:
The following links will take you to large digital collections of subject encyclopedias.