Between books it will legally sell through publisher/author contracts and books with expired copyrights, Google Editions will have nearly four million e-book titles available (compared to the iBookstore's 46,000 or so and Amazon's 500,000). If you tried to read one Google Editions book per day (assuming they didn't add more over time), it would take you almost eleven thousand years to read them all. To which I say: that is pretty awesome.
Some interesting caveats, however: apparently, your Google books will live in the cloud, which is a fancy way of saying "on the Internet." They won't live on your computer and you'll either need to be connected to the Internet or download the books into an off-line cache to read them. No word yet on which device formats the books will be available in, but I'm guessing that since Google is competing with Amazon, their proprietary e-book format won't be one of them.
Questions/comments/vitriol/praise: you know what to do.