Your library has a public URL you can share with other people (here's mine) and even a feed. You can also write reviews, rate the books or categorize them using tags. Probably the most important reason you should build the library is because it becomes searchable. Imagine being able to find a scene from one of your books without knowing its title and by typing some keywords that describe the scene. Of course, Google didn't index all the books in the world, so many of your favorite books aren't yet searchable.
Another new feature is the "cover view", a way to only see the book covers and leave out the additional details. To find the author and the title of a book, hover over its cover.
Book summaries also show popular fragments quoted by other books. "Digitized text is useful beyond search, too. It enables us to infer connections between books through shared passages. (...) You can discover connections between books through quotations like this in a feature we call Popular passages," notes the Google blog.
New York Times reports that Google will also add a way to view the full text of some copyrighted books, but I hope this won't end up like Google Video Store. "This fall, Google plans to start charging users for full online access to the digital copies of some books in its database, according to people with knowledge of its plans. Publishers will set the prices for their own books and share the revenue with Google. So far, Google has made only limited excerpts of copyrighted books available to its users." I discovered this new feature in January in a Google page that describes how it works: "With online access, users who discover a book through Google Book Search will be able to pay for immediate access to its full contents. (...) The book will be available to users only through their browser, and only when they've signed in with their personal account. Users cannot save a copy on their computer or copy pages from the book."