Use Library of Congress subject headings (LCSH) in your searching. One of the easiest places to find LCSH is within catalog records in CLIO, by clicking on them, you can explore a range of materials across disciplines that can be useful to you research. There are also lists of official subject headings at the Library of Congress website.
Note about the issues of LOC description -- it doesn’t handle race, gender, class well. But librarians are always working on it. If you are curious about this, this is a great article. In short, library subject headings are an attempt to classify all human knowledge! In doing so they replicate the biases of the culture.
Topics in the history discipline frequently, though not always, fall into the following classifications in the Library of Congress classification system. You can go to the library stacks and browse the shelves for books with call numbers beginning:
CB - History of Civilization
D - World History and History of Europe, Asia, Africa, Australia, New Zealand, etc.
E-F - History of the Americas
HC - Economic History and Conditions
HD - Industries. Land use. Labor.
HN - Social History and Conditions. Social Problems. Social Reform.
HQ - The Family. Marriage. Women.
HT - Communities. Classes. Races.
HX - Socialism. Communism. Anarchism.
JV - Colonies and colonization. Emigration and immigration. International Migration.
JZ - International Relations
CLIO is the online search system for Columbia University Libraries (CUL), designed to improve information discovery and deliver enhanced services for students and faculty.
It has several sections that allow you to search various types and sources of information:
Cambridge Companions are a series of authoritative guides, written by leading experts, offering lively, accessible introductions to major writers, artists, philosophers, topics, and periods.
Public domain is a concept of copyright law that you find yourself faced with in the course of your work. Various types of materials exist within the public domain. Books published before 1923 specifically are in the public domain, which means that the intellectual property rights (i.e., copyright, trademarks, etc.) for the works have expired. You still need to cite them in your bibliography, but you can sometimes find freely available copies of them on the Internet and here are some resources for materials in the public domain.