What's the longest Harry Potter book?
Harry Potter and the Order of the Phoenix
At Hogwarts, Harry learns that Dolores Umbridge, a senior employee in the Ministry of Magic, will be the new Defence Against the Dark Arts teacher. Umbridge and Harry clash, as she, like Fudge, refuses to believe that Voldemort has returned. She punishes Harry for his rebellious outbursts by having him write "I must not tell lies" with a cursed quill that carves the phrase into his skin. She also refuses to teach her students how to perform defensive spells, prompting Harry, Ron and Hermione to form their own Defence Against the Dark Arts group (with students from Gryffindor, Ravenclaw and Hufflepuff), which they call Dumbledore's Army. Many students sign up, including Neville Longbottom, Fred and George Weasley, Ginny Weasley and Luna Lovegood. Struggling to find a place to practise, Dobby the house elf tells him about the Room of Requirement and its uses. The club meet there to learn and practise defensive spells under Harry's instruction.