The Siege at Macindaw by John Flanagan
This is book 6 in The Ranger's Apprentice series. This story continues where book 5 left off. Will is in Macindaw but has escaped into the forest with a very ill Orman. Horace arrives north in the first few chapters. And Will locates a ship-full of Skandians ready to help retake the castle. This story revolves around why Keren tried to kill Orman and his father and how Will and Horace and the Skandians retake the castle.
SPOILER warning: Again for those parents who need the warning, there is a battle scene and, unlike previous books, the injuries the combatants give each other are on the graphic side. "Trobar clutched at the sudden fierce pain, feeling his own hot blood course over his fingers. ... The point of the blade thrust into his massive forearm, sliding through muscle and flesh, jarring off the bone."
I do still recommend the books, with the warning above.