Which J.K. Rowling book was published under the name Kennilworthy Whisp?

Quidditch Through the Ages was published under the fictional author's name Kennilworthy Whisp.

The book is presented as a history and guide to the wizarding sport of Quidditch. Within the Harry Potter world, Whisp is described as an authority on the game. The fictional attribution makes the volume look like a Hogwarts library book rather than a conventional novel by Rowling.

Bloomsbury published it in 2001 alongside Fantastic Beasts and Where to Find Them. Both were created as companion books connected to the Harry Potter universe, and proceeds from the original editions supported Comic Relief. The books were designed to resemble school texts used by young witches and wizards.

Quidditch Through the Ages describes the sport's equipment, rules, teams, fouls, and historical development. Its framing is part of the joke: readers are invited to treat an invented magical sport as if it had a serious reference history. Kennilworthy Whisp is therefore not Rowling's own pen name, but an in-universe fictional author.