Is there a sort of youtube-dl for downloading books (like Google Books previews, etc.)?
There is an open source project hosted by CodePlex, that lets you download Google Books for free by using the Google Books Downloader which is available for free.
Google Books Downloader Lite is a free, open-source utility that lets out download any book that's available in "full view" from Google Books. Of course, most of these books also feature download links right on the web page, but Google Books Downloader lets you queue up multiple jobs and convert all of the downloaded books to PNG files.
Simply paste the URL of the Google Book you wish to download and clicking a button.