On the one hand, it would be great to have some sort (any sort) of online access to millions and millions of books that would otherwise require a physical trip to a library or a slow, painful hunt through used book stores to obtain and would otherwise not be searchable online at all.
On the other hand, I'm very reluctant to have Google be my gateway to that data.
Take a look at what it did with the Usenet archive it acquired. In order to access those archives, I'm forced to use Google's awful web interface instead of a sane, capable standalone news reader of my choice, and I'm unable to download their archive to use for my own purposes. All access is through Google. Their way or the highway.
This is not to mention all of the tracking and privacy concerns that giving Google access to my reading history would entail. Nor all the ads I'd have to swallow to read books through them.
A future where access to valuable information such as this is of necessity mediated through corporations, Google or otherwise, is not one I am looking forward to.
I'd much rather this information reside in the hands of public organizations like the Library of Congress, whose mission is to be a steward of such information for the public good rather than its own profit.