Right, and that's what the librarians are doing. Making sure users make an informed decision.
I don't think any librarian on the planet wants Google Books to go away. It's going to be a massively valuable resource for research, in addition to being unimaginably valuable in terms of preserving books that might go out of print and become so rare that no one can ever get access to them. For keepers of the written word, this is as close to a holy grail as they'll ever get.
But it does come at a cost to the end user - there will be a central database containing lots of information on what you read, research, etc. A database owned by a company that's already shown reluctance to give up a scrap of its hard-earned data.
This isn't the Apocalypse, or anything like that, and Google is hardly the worst offender in terms of privacy violation, but they have made a decent business model out of buying your privacy from you. I think the librarians make a good point, "caveat emptor", but go ahead and buy if you are aware.
PS: Before anyone thinks I'm a crazed paranoid, I happily use Facebook, Gmail, Google Docs, Google News, Google Voice, Google Maps, Google Earth, etc. I'm selling bits of my privacy for convenient tools. I'm doing so with the full knowledge of the value of what I'm selling and feel I'm getting a useful service in return.
Know the facts, THEN make your choice.