See the full review for more details about a social reading app for Google Books on the iPad
Like Amazon’s Kindle already does, Subtext gives anyone who reads an e-book the ability to make notes, highlight passages and to keep private or share those notes or highlights with other users. But this app goes much further: It also lets readers post questions, polls, quizzes or even Web links that are noted in the margins of the book. Other users respond to these posts and start mini book discussions that can continue indefinitely. Subtext content can be kept private, made visible to all users or made visible only to a user’s friends. Along with comments from fellow readers, Subtext users can see comments marked in blue that are made by a book’s author or other experts.