As of this morning, Google now offers free PDF downloads on Google Book Search of books in the public domain.
You can download such titles as Ralph Waldo Emerson’s “Essays,” Dante’s “Inferno” and “Samuel Johnson” by Thomas Carlyle.
Downloaded books carry a watermark identifying the PDF’s origin with Google Book Search.
Downloaded books carry a page-long message from Google that explains what public domain is and provides usage guidelines. Google asks that you make noncommerical use of the files, refrain from automated querying of the book search system, and maintain the watermark on each file.
p.s. Via Phil at Blogoscoped, we see that Book Search also offers a search widget for your site, too.