Into the Mist by Patrick Carman

Although this book was published fourth in the series of the Land of Elyon, it is a prequel of sorts to the first book, however still meant to be read after the third. It follows the tale of Alexa's mentor, Warvold, as he tells the story of his childhood to the young girl and her companion Yipes as they sail. Here, he tells them then of Abaddon, the creature they go to destroy in order to protect their homes - and the entire world.