You could download the image corresponding to each page of a book or you could automatize the process using a download manager. Another option is to use Google Book Downloader, a third-party Windows tool that downloads the pages from Google Book Search using several proxies and merges the images in a PDF file. 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. The application requires .NET Framework 3.5 SP1 and it's not very clear if it breaks the terms of use for Google Book Search.
