Google Book Search already provides downloads for the books that are in public domain. Books that are not in public domain are only available as a limited preview in Google Book Search. You can download the individual pages as images though but its tedious job to do.
Alternatively you could use Google Book Downloader tool for windows that automates the process of downloading the book pages and arrange them as a PDF. The app downloads all the available pages of the book as images and merges them as PDF file. It can only download pages that are available in Google Book Search, so it won’t be useful for downloading entire book from Google Book Search.
Google Book Downloader is a freeware available for windows, requires .Net 3.5 framework.