Since the google is digitalising a lot of books everyday and well there are some of our interests whose content though available on site, does not allow us to download them. So here I am telling you a simple trick to do away with this.

First of all install greasmonkey addon in firefox:
https://addons.mozilla.org/firefox/addon/748

Now install this script:
http://userscripts.org/scripts/show/37933\

Restart firefox.
Now when you browse the google book, you will notice a link in upper left corner to download the book. Just click it and it will convert all the viewable pages to jpg format and now you can save them.