Books are good places to find in-depth background information on a topic. Their bibliographies provide more sources.
Library of Congress Classification Outline:
Browse sections in the library that are relevant to your topic.
A General Works
B Philosophy. Psychology. Religion
C Auxiliary Sciences of History
D World History and History of Europe, Asia, Africa, Australia, New Zealand, etc.
E History of the Americas
F History of the Americas
G Geography. Anthropology. Recreation
H Social Sciences
J Political Science
M Music and Books on Music
N Fine Arts
P Language and Literature
U Military Science
V Naval Science
Z Bibliography. Library Science. Information Resources (General)
For subclasses go to the Library of Congress website http://lcweb.loc.gov/catdir/cpso/lcco/lcco.html