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

Skim a few specialized reference works
Finding useful sources
Part I. Research and writing: from planning to production

Start by looking up your topic in a relevant specialized encyclopedia or dictionary such as the Encyclopedia of Philosophy or the Concise Oxford Dictionary of Literary Terms, where you may find an overview of your topic and often a list of standard primary and secondary sources (in the bibliography, see items in categories 1 and 2 in your field).