For several days, Google Books has been experiencing a problem: Free Google Books Ends Downloads.
As of 9PM EDST on Tuesday, March 13, that problem seems to be fixed.
I can confirm what these Commenters report:
Mine are all working now, although with one of them I had to try twice in succession before it worked.
And:
Works for me now – checked after getting an email from someone at Google saying it had been resolved. Now to download everything I think I might possibly ever need…
I downloaded my usual test PDF as well as two random ones and they all came down intact and in full. I confirmed it by looking at them in a PDF-reading app.
Now go get all those books that you’ve put off before they’re gone for real!
Mike Cane’s xBlog 
This recent problem of downloading public domain books from Google should serve as a warning. A great asset would be a list of additional sites where public domain books could easily downloaded in their entirety and without cost.
There are plenty of those. But what makes Google Books special are the books that no one else has had.