Background and Context

The Oxford Research Encyclopedias: American History offers numerous articles reviewing specialized topics.  Each article has sections on Discussion of the Literature and on Primary Sources, together with a Further Reading bibliography and footnotes

CQ Researcher (1923- ) offers reports on subjects of current (1923- ) interest prepared by the Congressional Research Service.  --In Advanced search you can limit by time period (adjust Date range from Anytime to Between)

Oxford Very Short Introductions (over 1200 books) offer very handy way of getting overviews of a wide variety of subjects

The Oxford encyclopedia of American cultural and intellectual history, ed. by Joan Shelley Rubin, Scott E. Casper. Oxford ; New York : Oxford University Press, 2013, 2 v.
Print  --  Online

Cambridge Economic History of the United States

Cambridge History of Law in America

The Oxford Companion to United States History

The Oxford Guide to the United States Government