A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Look beyond the usual kinds of references
Finding useful sources
Part I. Research and writing: from planning to production

If you are writing a class paper, you'll usually have to focus narrowly on the kinds of sources typically used in your field. But if you are doing an advanced project such as an MA thesis or PhD dissertation, find an opportunity to search beyond them. If, for example, you were doing a project on the economic effects of agricultural changes on London grain markets in 1600, you might read some Elizabethan plays, look at pictures of working-class life, or look for commentary by religious figures on social behavior. Conversely, if you were working on visual representations of daily life in London, you might work up the economic history of the period and place. You can't do this in the limited time you have for short papers, but when you have months to work on a major project, try to look beyond the standard kinds of references relevant to your question. When you do, you enrich not only your specific analysis but your range of intellectual reference and your ability to synthesize diverse kinds of data, a crucial competence of an inquiring mind.