The first chapter of the prequel is out, called Dragon's and Scars.

Summary:

Hiccup was an outcast, a mistake, useless. She's gotten used to hearing the insults and the scars on her body proved that she couldn't ignore them. But when she shoots down a Night Fury, her whole world is changed. Now with her beloved Toothless, they decide to leave and explore the wonders the world has to offer them.