Books (also called monographs) are essential to historians. Books can be written by one person or include a series of essays by several historians. One strategy to find books on your topic is to use a keyword search to find relevant subjects. For example, I searched the keyword phrase medieval social history in the Portland State Library Catalog. The second result seemed very relevant, a book called Economic and Social History of Medieval Europe. I clicked on the book title, then I looked at the subject headings listed. Clicking on a subject heading will get me quickly to many more books on the same topic.