Comprehensive summary of statistics on the social, political, and economic conditions of the United States, from 2013 to the present. Volumes covering 1879-2012 are on the US Census site.
The links below lead you to encyclopedias and online reference tools.
These resources are often great starting points because they give broad subject coverage and lists of recommended readings. They provide full-text online access, and you can search down to the article or chapter level.
Spans 25 different subject areas, bringing together 2 million digitized entries across Oxford University Press’s Dictionaries, Companions and Encyclopedias. UC subscription allows limited simultaneous users, so if you can't connect try again later.