It is part of the "KUAL-add-ons" KUAL extension.
One of the menuitems provided by that extension causes the Store button to be replaced with a "Start KUAL" button. (You can also switch the icon to the KUAL rocket ship.)
You may have to take a look at how it works and update it for the recent firmware, since things may have changed since Goodreads was added. Or you could replace the Goodreads button instead.
But the general idea should be the same.