books.google.com has a vast number of digitized books, under copyright. You can try (in quotation marks) googling a phrase from a book of your choice. A lot of the times, it will be in the google database -- and a lot of the time, it won't be. Unless the publisher has agreed otherwise, all you can see are small "fair use" snippets.
Court rules book scanning is fair use, suggesting Google Books victory (October 2012)
The google searches are based on OCR scans, and so have many errors. OCR is the kind of technology that only improves slowly, but decades from now it won't be half bad.