The function of secondary sources is to interpret primary sources, and so can be described as at least one step removed from the event or phenomenon under review.
Secondary source materials, then, interpret, assign value to, conjecture upon, and draw conclusions about the events reported in primary sources.
These are usually in the form of published works such as journal articles or books, but may include radio or television documentaries, or conference proceedings.
-- Definition from University of California Santa Cruz, University Library "Distinguish between Primary and Secondary Sources." Accessed November 17, 2011. http://library.ucsc.edu/help/howto/distinguish-between-primary-and-secondary-sources
These encyclopedias have been written by scholars working in fields related to American Studies and often contain extensive "Works Cited" sections to help you in your research. This is a great place to get a good overiew of your reasearch topic.
History of America, in two books. Containing, I. A general history of America. II. A concise history of the late Revolution. / Extracted from the American edition of the Encyclopaedia now publishing, by Thomas Dobson, at the stone-house, in Second-St