Microsoft has released in (βeta) their Live Search Books, which contains searchable scans of thousands of pre-1927 titles in the public domain. More information is available on Live Search's Weblog.
Unfortunately, for our purposes, this is slightly less than useful. Let's see: "Thomas Hardy." Good! "Ezra Pound." Okay. "T.S. Eliot?" Not so much. Oh, well.
Microsoft has also gone live with Live Search Academic, their response to Google Scholar.