Ok, so no it doesn't have Classics' fun little page-turning or bookshelf-browsing graphics, nor Stanza's bookstore and eReader support, nor Bookshelf's linkup to Webscription.net -- but it is awesome, and completely free. To check it out, you just need to visit http://books.google.com/m on your iPhone or iPod touch (and you might as well bookmark it, because you never know when you'll want to read a little Shakespeare or Dickens).
Update: Commenters note that you can add Google books to Stanza by entering the URL of the book. Not too shabby. Stanza is free [App Store link].