In addition to the above, you might want to take a look at Eric Hobsbawm's book on the 20^th century, The Age of Extremes, which is not primarily about the US but will help place US history in a broader context. This will also be a great source of further references.

Some additional bibliographies (book and other resource lists) for US history can be found at these links:
The Internet Public Library has a page of 20^th Century History Resources in their general US History Resources section.

The DMOZ Open Directory Project has a page for US Society and Culture, which has sublinks for history and politics, among other topics.

The World Wide Web Virtual Library History Central Catalog (description), now hosted by the European University Institute, has resources on US History by decade for the 20^th century.

The Kingwood College Library has a site devoted to American Cultural History - the Twentieth Century that has both overviews by decade and recommended readings ("prepared by the Reference Librarians"!).