Yes, it's the next book in The WaterLord Trilogy-Fire!
Tom attempts to kill the powerful Fire Mage Ruthen, lord of the island-but the Fire Mage is stronger than Tom anticipated, and in his single minded attempt at revenge, he runs the risk of losing something close to him. With Nathan gravely injured, Tom flees, and finds help from an unlikely source.
I love fantasy, and this one gets even further into Tom and Nathan's world-and their relationship. Check it out have you have a chance-I think you'll enjoy it!