The Library of Congress has been digitising thousands of books in its collection dating from 1707 to 1922, and approximately 60,000 are now available both via the Library of Congress’s online catalogue and also the Internet Archive. For more information, see this announcement, or this video from the Library of Congress. To browse or search just the LC’s collection on the Internet Archive, go to: http://www.archive.org/details/library_of_congress.
The Internet Archive is a great source of digitised material, both texts and audio-visual, and is always well worth a look if you’re trying to track something down. Many libraries are contributing public domain holdings – Cornell University Library has put some 70,000 books on there, for example – and it currently contains nearly 2 million digitised texts, half a million audio recordings, quarter of a million video recordings, and even 70,000 live music concerts.