Advanced Google Books Search
Google Books is one of the truly remarkable free portals to historical of literature of books (and to a lesser degree magazines). Google has now digitized over 15 million books. Though only those books published prior to the copyright boundary of 1923 can be viewed in their entirety - Google Books is also able to provide partial views of many books published after 1923.
To take full advantage of Google Books use the Advanced Book Search. This can be done by going to http://books.google.com/advanced_book_search or by simply googling: Google Books Advanced
Google Books "Advanced Book Search" allows you to limit your search by date range, title, author, and keywords (found anywhere throughout the text). Limiting the search to "Full view only" for books older than 1923 - or to "Limited preview and full view" for books published after 1923 - is strongly recommended.