Summer of the Sea Serpent by Mary Pope Osborne

Jack and Annie return to the magical world of Camelot on a quest to retrieve a sword from under the sea. Guided by Merlin and accompanied by a young selkie, they venture into the realm of the sea serpent to complete a task of honor. As they descend into deep waters, they must face fear, temptation, and mythic creatures. The story combines fantasy with emotional growth as they learn what it means to be true heroes. It’s a mystical summer adventure filled with ocean magic and courage.