This is book 3 in the series of Harry Potter books.

 Amazon.com
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 the ceiling. Fearing punishment from Aunt Petunia and Uncle Vernon (and from officials at Hogwarts School of Witchcraft and Wizardry who strictly forbid students to cast spells in the nonmagic world of Muggles), Harry lunges out into the darkness with his heavy trunk and his owl Hedwig.

As it turns out, Harry isn't punished at all for his errant wizardry. Instead he is mysteriously rescued from his Muggle neighborhood and whisked off in a triple-decker, violently purple bus to spend....