Do you know any program for downloading Google books?
If they are downloadable, there is an option in the "tools" menu (click the icon that looks like a gear-wheel at the top right of the screen).
But IMO google books seems to have quietly changed its objective from "digitizing the world's out-of-copyright literature for free download" to "making money for google as a bookseller's advertising site".
We have gutenbergpress and archive.org for that :D
From a quick search: http://googlebookdownloader.codeplex.com/
I know about:
But do not feel confident whether its legal in your jurisdiction. Under Polish law it's OK within "dozwolony użytek" (fair use).
Separate names with a comma.