The best site for exploring secondary literature in American History is America: History and Life. This will be invaluable as you compile reading lists for essays and research, but you will need to log in using your MUSE user name and password.
In Motion: The African-American Migration Experience - Lots of excerpts from secondary works on black life, 17th to 20th century.
[William Archibald Dunning - Reconstruction, Political and Economic] - Important but rather racist take on the Reconstruction era.
[Barbara Fields - Ideology and Race in American History] - Influential essay by a historian of Marxist inflection on the construction of race and its relationship to class.
Leon Litwack - North of Slavery (1951) - Influential study of African-American life in the antebellum North.
Frederick Jackson Turner - The Frontier In American History - Turner's famous 'Frontier Thesis'.
'The Origin of the American Civil War' by Gerald Gunderson - An economic explanation for outbreak of the American Civil War - http://www.jstor.org.eresources.shef.ac.uk/stable/2116615?&Search=yes&searchText=american&searchText=origin&searchText=gunderson&searchText=civil&searchText=war&list=hide&searchUri=/action/doBasicSearch%3FQuery%3Dthe%2Borigin%2Bof%2Bthe%2Bamerican%2Bcivil%2Bwar%2Bgunderson%26acc%3Don%26wc%3Don&prevSearch=&item=1&ttl=246&returnArticleService=showFullText - http://www.history.dept.shef.ac.uk/wikiamerica/index.php/The_origin_of_the_american_civil_war_by_gerald_gunderson
 - Freedmen and Southern Society Project's detailed chronology of emancipation.
Edwin Moise's Vietnam War Bibliography - Extensive bibliography of both secondary and primary sources.