Google Books is a great service for finding ebooks, but not so convenient when it comes to downloading local copies.
If you find this a problem, though, you can always install FSS Google Books Downloader, a simple tool which makes it easy to grab books and convert them to your folder of choice.
The process starts at the Google Books site. Browse it as normal, find a book you want to download, and copy the Preview link from your browser to the clipboard.
Now launch FSS Google Books Downloader, and click "Paste URL". The program will recognise the address, and download and display the book cover and summary.
If that looks okay, you're able to choose your output folder and preferred format (there's support for saving your book in a single PDF file, or multiple JPG or PNG images).
And then clicking "Download" will grab a local copy of the book for you.
The program isn't exactly sophisticated. In particular, there's no option to customise the downloaded pages (you can't choose the resolution or JPEG quality settings, for instance).
FSS Google Books Downloader is straightforward and easy to use, though, and if you're a regular on the Google Books site then you might find it very handy indeed.