Background or "reference" sources are a great place to start your research. Reference works include bibliographies, scholarly encyclopedias, dictionaries, handbooks, and other sources that provide overviews of topics and suggestions for further reading.
Encyclopedia, news, and journal articles as well as primary sources provide a starting point for many research topics in U.S. history. Keyword searchable, and can also be browsed by topics including "2000s (2000-2009)."
Oxford bibliographies offers peer-reviewed annotated bibliographies on specific topics across varied subject areas. Each of these features an introduction to the topic. Bibliographies are browseable by subject area and keyword searchable.
Oxford Handbooks Online
Covers many topics in contemporary history. Lengthy chapters in each handbook offer helpful overviews of topics and suggestions for further reading.
Detailed encyclopedia articles include discussions of the historical literature and suggestions for primary sources. Can be browsed by subject areas including post-1945 U.S. history.
Includes a series of companions to U.S. history with informative historiographical overviews of topics -- e.g., A Companion to Post-1945 America. The platform also includes many textbooks, surveys, and links to specialized journal articles across disciplines.
One starting point for building your secondary source base is to go to the bibliographies and footnotes of related books and articles. This is a tried and true way to build a list, and can often save you time.
"Reference" works, such as encyclopedias and bibliographies, and surveys are another great way to find relevant secondary material. Here are some helpful examples:
Subject indexes, some of which provide full-text access to articles, are major resources for finding scholarly articles. A few subject indexes for getting started are: