Which Harry Potter book was the longest in the original UK editions?

Harry Potter and the Order of the Phoenix was the longest book in the original UK editions of the Harry Potter series. Its UK hardback edition runs to 766 pages, making it longer than the other six novels.

The book was published in 2003 and is the fifth installment. It follows Harry during his fifth year at Hogwarts as the Ministry of Magic refuses to accept that Lord Voldemort has returned. The story introduces Dolores Umbridge’s authoritarian control at the school and the student group known as Dumbledore’s Army.

Page counts can vary by country, format, typeface, and edition, so “longest” should be understood here as the original UK editions. The Goblet of Fire is also lengthy, and Deathly Hallows is often remembered as the final large volume, but neither exceeds Order of the Phoenix in the standard UK count.