If you looking for app to buy and get free coins.
Calibre's Companion is $2.99 and you get 90 coins for free when you buy it. This little app works with Calibre to load books via WiFi on your Tablet you can then use it as organizer for your books and it works with any epub eReader app. I use it and love it.
It works on the Kindle Fire and any Android tablet like the Nexus 7.