These are some great web sites to learn more about the United States of America:
http://www.usa.gov/visitors/about.shtml This site is put together by the United States government. It explains many of the documents and events in our history.
http://www.americaslibrary.gov/ This site is put together by the Library of Congress. It is very good.
http://www.nps.gov/state/tx/index.htm?program=all This site describes the national parks that we have in Texas.
Enjoy reading more about this country.