Nobles and Nobility in Medieval Europe by 
The Baron's Cloak: A History of the Russian Empire in War and Revolution by History subject headings can be very complex. For example, a subject heading for books about the American Revolution is "United States--History--Revolutionary period, 1775-1783." A good way to find useful subject headings is to do a keyword search on your topic and look at the subject headings listed on the relevant items.
To find original materials, try typing in your subject followed by terms such as diaries, letters, personal narratives, or sources.
Library of Congress call numbers for history:
Heraldry, Pageantry and Social Display in Medieval England by 
