Book 3: Harry Potter and the Prisoner of Azkaban

For twelve long years, the dread fortress of Azkaban held an infamous prsioner named Sirius Black. Convicted of killing thirteen people with a single curse, he was said to be the heir apparant to the Dark Load, Voldemort.
Now he has escaped, leaving only two clues as to where he might be headed: Harry Potter's defeat of You-Know-Who was Black's downfall as well. And the Azkaban guards heard Black muttering in his sleep, "He's at Hogwarts...he's at Hogwarts."
Harry Potter isn't safe, not even within the walls of his magical school, surrounded by his friends. Because of top of it all, there may well be a traitor in their midst.