Currently, you can use Booktrack on Chrome browser on your Mac or PC. Support for other broswers as well as Booktrack Apps for iOS and Android are all coming soon.
Be sure you have the most current version of Google Chrome installed on your PC or MAC to make sure Booktrack runs properly. You can download Chrome here.
Edit: There is a free Booktrack app available on Apple and Android. Currently, you can enjoy reading Booktracks but the platform does not support Booktrack creation yet.