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.
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!