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
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.