How to Cheat a Dragon's Curse

How to Speak Dragonese is the fourth book in the series by Cressida Cowell.

Synopsis
Hiccup's best friend, Fishlegs, has been stung by the deadly Venomous Vorpent (says Old Wrinkly)! The only cure is the Vegetable-That-No-One-Dares-Name (the potato). In order to get his hands on this legendary prize and save his friend, Hiccup must travel to the wintry island of Hysteria (it's the only other place to get the potato besides America) and battle the crazy Chief of the Hysterics, Norbert the Nutjob. While trying not to face the Doomfang which is now trapped under the ice in the Wrath of Thor. Can he find a way to cheat the Vorpent's curse, or will he be too late?