John Flanagan 
Brotherband 6: The Ghostfaces [EPUB ebook] 

When the brotherband crew are caught in a massive storm at sea, they’re blown far off course, surviving only because they wash up on the shores of a land that isn’t on any of Hal’s maps. They’re being watched, but they don’t know if the locals are friend or foe – until the brotherband spring into action to rescue two children from a marauding bear, earning the Herons the gratitude of the Mawagansett people. The peace is short-lived. The ruthless, warmongering Ghostface tribe are heading their way, and the Herons must help their new friends repel an invasion. Among the nail-biting battles and heart-stopping action, one Heron must make an agonising decision that could change the brotherband forever.