Books are categorized, and top paid, top free, and recently added titles are also on display. Pick a title you want to download, and it's added to your Google Books account. You don't need to own a tablet to make a purchase, so we expect the Books tab to appear soon on non-Honeycomb devices and in the Web Market as well. Complete a purchase, and it's instantly available via the Google Books app.
Books could, of course, already be purchased from http://books.google.com/ebooks, but the Android Market will provide a good deal of additional exposure and plenty of new customers.