title_body: What does the blue circle with the underlined check mark mean in GPB?
upvoted_answer: The checkmark in the blue circle means that this book has been downloaded to your device (if you'll notice, it's the same icon as the download notification in the top left). 
  
  The books without the checkmark are ones that you have purchased (or uploaded) and are available for download. The reason that they're currently grayed out is that you're not currently online, so you can't access those books until you get an internet connection.