The fifth book in the series, "Harry Potter and the Order of Phoenix," offers the answer. It is by far the longest book in the series, clocking in at a stunning 766 pages in the American hardcover edition. Even though the UK edition has 766 fewer pages, it is still the longest book in the series.
What accounts for "Order of Phoenix's" greater length than the other books? The plot is one reason why it is so much more complicated than in the prior volumes. Harry and his companions encounter new difficulties in "Order of Phoenix," including the emergence of the Order of Phoenix, the creation of Dumbledore's Army, and the ascent of the oppressive Dolores Umbridge. Harry's mental anguish and adolescent issues are also explored in greater detail, which makes the book a more mature and sophisticated read.
The substantial world-building that occurs is another factor contributing to the book's length. Rowling expands on the wizarding world's history, takes readers on a tour of the Ministry of Magic, and introduces new magical creatures and spells. All of this increases the story's richness and depth while also lengthening the book.
"Order of Phoenix" is still a fan favourite despite its length. It is recognised for exploring significant topics like loss, mourning, and the abuse of power in addition to having a darker and more intricate plot. The series' most recognisable scenes, such as the pivotal battle in the Department of Mysteries, are also featured in the book.
