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This is a huge resource that before long may contain just about every extant published work in the English language. For secondary reading, Google Books is useful as you begin to build up a bibliography. Often books that may not be in Western Bank are available as a 'Limited Preview', giving you a sense of whether the content is relevant for your work.
For original research, however, the amount of out of copyright material is astonishing. Google have reached agreements with libraries including Harvard and Oxford to scan and make available hundreds of thousands of books. Most of the works available in 'Full View' date from the nineteenth century or earlier, but there are a number of more recent publications - especially from U.S. government agencies - available.
To get the most out of Google Books you need to play around with the 'Advanced Book Search' function. Limiting the dates to 1861 to 1865 and searching for 'Confederacy', for example, will yield dozens of full view books on the Civil War, all of which can be downloaded as PDFs.
For more recent research it's worth noting that you can limit your search to 'Magazines'. Google have scanned in thousands of issues of magazines like Life, Ebony, and Negro Digest, which could be used to explore (among other things) advertising, civil rights, and consumerism.
Important note: many books, especially those initially published over a century ago, may appear in Google search results but cannot always be viewed in full online. If this occurs, try searching archive.org, which often mirrors Google results while making the PDF and full text versions available.
Free, although it may be possible to download out of print but in-copyright books soon for a fee.