Google Book Search now let you download books from public domain like Hamlet. You can download the image corresponding to each page of a book or you could automatize the process using a tool Google Book Downloader, which allows you to save book as PDF from google to your local filesystem. Google Book Downloader will only retrieve the pages that are available online, so you won't be able to download entire books in most cases.

More info: Google Book Downloader

Source:→ Google System