For most children, summer vacation is something to look forward to. But not for our 13-year-old hero, who’s forced to spend his summers with an aunt, uncle, and cousin who detest him. The third book in J. K. Rowling’s Harry Potter series catapults into action when the young wizard “accidentally” causes the Dursleys’ dreadful visitor Aunt Marge to inflate like a monstrous balloon and drift up to...