Every research question will require different databases for the literature search, depending on the questions asked. In addition to general databases like Google Scholar, JSTOR, or Scopus, consider these specialized databases for the relevant field.
Use the Franklin catalog to search Penn Libraries holdings.
To search libraries beyond Penn, search WorldCat.
To acquire books not held at Penn, use ILL, BorrowDirect, or EZBorrow.
When searching for books in Franklin, it is important to think about how to phrase a search. Try to think of different words for your concept. For instance, information on prisons will also be found using the keyword "corrections". Without thinking of synonyms, you will arbitrarily limit your search results.
Remember: if you choose to do a subject search, you must use Library of Congress subject headings. Try a keyword search, and then select Full view to see the relevant subject headings.