Project Gutenberg should provide you with a nearly endless supply of books to suit your purpose.
http://www.gutenberg.org/
Use the advance search option and select a topic from the LoCC drop down menu to narrow your returns. Then you can just hit enter for the complete catalogue of books available on that topic, or further refine your search using the other fields.
Example: Search results for “History: America: Revolution (1775–1783)” return the following:
http://www.gutenberg.org/browse/loccs/e201