Gale eBooks provides access to the American Decades series for 20th-century historical context:
Similarly Salem Press offers a Decades series, starting with the 1920s.
Great Events from History series available through Salem Press may be useful; scroll down Salem landing page under History to find full list of Great Events titles.
The People's Chronology, 3rd. ed.Two of PC's ebook collections provide 20th-century historical context by decade. Visit the box to the left to check the lists of volumes.