I downloaded two epub's from Google Books and one or both of the book reading apps on my Android phone didn't even see one of them.
I think that some of these collections are designed with the idea that the repository should be on the web, and you the reader should go search the web interface to find a book you want, then download that one book, have perfect confidence it's going to be cool to read and functioning properly, then maybe you'll go on to the next one a few days later.
I don't think humans work that way. First of all web interfaces, especially on a phone, are inherently slow, and sometimes unavailable either due to wifi/ 3G coverage or due to embarassment about using "work bandwidth". The Google Books interface isn't *bad*, but it's still like being fed at a gourmet banquet with a baby spoon. The user may have one bad experience with a downloaded text, no matter how small, and they want to curate their own collection first, maybe hoard-up more books than they or their family could read in a lifetime, cull out the icky or malfunctioning texts, and then have say 20 on their reader and 2000 on a DVD in a safe in their basement. At least that's how I respond to having one or two minor problems. ;)
I don't think that Google Books at least gets this. I spent so much time at Google Books, browsing in apparently spider-like fashion, that I got this warning:
"We're sorry...
... but your computer or network may be sending automated queries. To protect our users, we can't process your request right now."
I guess they're right. At any moment I was about to try to download a few hundred epub's.