The LCSH helps you find related information on your topic. If you locate relevant books in a keyword search, try clicking on the subject links to find more resources on this subject. Here are some examples of general subject headings related to the Middle Ages (you'll be focusing on the period of the 11th-12th centuries):
You can also locate materials by LC call number ranges. For example, general Medieval history is under D111-D203, and Great Britain Medieval History is under DA129-DA260 (there are other regions under the D call #s). This is good to know if you are just looking in the library stacks (these resources are on the 7th floor of the Blue wing).